Embedded Developer

ESP8266 boards offer an amazing opportunity to explore the IoT world at a low cost. The boards start for as little as £5 here in the UK and come in a variety of form factors from one with only two GPIO pins to larger boards containing more GPIOs, PWM, I2C, SPI and analog capability. The boards started to become noticed around the middle of 2014 and have become more popular as the tools and documentation have matured.

Support for the boards is mainly through the online community forum. The latest documentation and SDKs can be found through the same forums.

The boards make low cost networks of sensors a possibility with the ESP8266 modules acting as either small servers or clients on your WiFi network.

The ESP8266 module is often supplied with one of two firmware version burned into the module:

1. nodeMCU – Lua interpreter
2. AT command interpreter

For the purposes of this exercise we will look at replacing the default firmware supplied with a custom application.

The SDK available from Expressif allows the developer to program the module using C. First thing to do is to install the tool chain for the module. The ESP8266 Wiki contains a host of information including instructions on setting up the tools on a variety of platforms. This whole process took about 2-3 hours on my Windows PC with another few hours to put together a template for a makefile application for Visual Studio.

Now that the development tools are in place we can start to think about the sensor network. Security is a major concern for Internet of Things (IoT) projects but this proof of concept will ignore this for the moment and rely on the WiFi encryption as the only mean of securing the data. This is a proof of concept afterall.

The applications developed here will allow data to be transmitted using UDL over the local area network.

UDP Receiver

Before starting on the ESP8266 application we should put together a simple application to receive the data from the local network. A simple .NET application on a Windows PC should do the trick:

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;
using System.Net;
using System.Net.Sockets;

namespace UDPConsoleClient
{
    class Program
    {
        static UdpClient _udpClient = new UdpClient(11000);

        static void Main(string[] args)
        {
            Console.WriteLine("UDP Client listening on port 11000");
            IPEndPoint _remoteEndPoint = new IPEndPoint(IPAddress.Any, 0); 
            while (true)
            {
                byte[] data = _udpClient.Receive(ref _remoteEndPoint);
                string message = Encoding.ASCII.GetString(data);
                Console.WriteLine("Message ({0}): {1}", _remoteEndPoint.Address.ToString(), message);
            }
        }
    }
}

Create a new Windows console application on the PC and add the above code to the program.cs file. When run, this application will listen to the local network and display any UDP packets as a string to the console. Nothing special but good enough for the purposes of this experiment.

Basic ESP8266 Application

The basic ESP8266 application works in a similar way to an Arduino application, there is an initialisation method user_init and a loop method. The loop method is slightly different from the Arduino loop in that it is added to a task queue and the developer is free to select the name of the method. A basic application looks something like this:

#include "at_custom.h"
#include "ets_sys.h"
#include "osapi.h"
#include "gpio.h"
#include "os_type.h"
#include "mem.h"
#include "user_config.h"
#include "ip_addr.h"
#include "espconn.h"
#include "user_interface.h"

#define USER_TASK_PRIORITY          0
#define USER_TASK_QUEUE_LENGTH      1

//
//  Callbacks for the OS task queue.
//
os_event_t userTaskQueue[USER_TASK_QUEUE_LENGTH];

//
//  User function which will be added to the task queue.
//
static void ICACHE_FLASH_ATTR UserTask(os_event_t *events)
{
    os_delay_us(10);
}

//
//  Initialise the application and add the user task to the task queue.
//
void ICACHE_FLASH_ATTR user_init()
{
    //
    //  Start OS tasks.
    //
    system_os_task(UserTask, USER_TASK_PRIORITY, userTaskQueue, USER_TASK_QUEUE_LENGTH);
}

Compiling and deploying this application will result in a module which does nothing much other than wait.

Debugging

Unlike some of the other modules and boards discussed on this blog, the ESP8266 does not have much available in the way of debugging. Code will need to be added to the application to output debug information on GPIO pins in order to get feedback on what is going on inside the application.

The ESP01 module being used has two GPIO pins available, GPIO0 and GPIO2. We can use these to output serial data akin to SPI and then use a logic analyser to examine the application state. Adding the code below will allow us to do this.

#define BB_DATA                     BIT0
#define BB_CLOCK                    BIT2

//
//  Initialise the GPIO pins.
//
void InitialiseGPIO()
{
    gpio_init();
    //
    //  Set GPIO2 and GPIO0 to output mode.
    //
    PIN_FUNC_SELECT(PERIPHS_IO_MUX_GPIO2_U, FUNC_GPIO2);
    PIN_FUNC_SELECT(PERIPHS_IO_MUX_GPIO0_U, FUNC_GPIO0);
}

//
//  Bit bang the data out as serial (SPI) data.
//
void BitBang(uint8 byte)
{
    //
    //  Set the clock and data bits to be low.
    //
    gpio_output_set(0, BB_DATA, BB_DATA, 0);
    gpio_output_set(0, BB_CLOCK, BB_CLOCK, 0);
    //
    //  Output the data.
    //
    short bit;
    for (bit = 7; bit >= 0; bit--)
    {
        if (byte & (1 << bit))
        {
            gpio_output_set(BB_DATA, 0, BB_DATA, 0);
        }
        else
        {
            gpio_output_set(0, BB_DATA, BB_DATA, 0);
        }
        gpio_output_set(BB_CLOCK, 0, BB_CLOCK, 0);
        gpio_output_set(0, BB_CLOCK, BB_CLOCK, 0);
    }
    //
    //  Set the clock and data bits to be low.
    //
    gpio_output_set(0, BB_DATA, BB_DATA, 0);
    gpio_output_set(0, BB_CLOCK, BB_CLOCK, 0);
}

Timers

For the proof of concept the application can simply generate a regular packet of data emulating the presence of sensor data. The most cost effective (in terms of power) way of doing this will be to use a timer of some sort.

//
//  OS Timer structure holding information about the timer.
//
static volatile os_timer_t timerInformation;

//
//  Timer function called periodically - try communicating with the world.
//
void TimerCallback(void *arg)
{
    BitBang(0x01);
}

//
//  Initialise the application and add the user task to the task queue.
//
void ICACHE_FLASH_ATTR user_init()
{
    InitialiseGPIO();
    //
    //  Disarm timer.
    //
    os_timer_disarm(&timerInformation);
    //
    //  Set up timer to call the timer function.
    //
    os_timer_setfn(&timerInformation, (os_timer_func_t *) TimerCallback, NULL);
    //
    //  Arm the timer:
    //      - timerInformation is the pointer to the OS Timer data structure.
    //      - 1000 is the timer duration in milliseconds.
    //      - 0 fire once and 1 for repeating
    //
    os_timer_arm(&timerInformation, 1000, 1);
    //
    //  Start OS tasks.
    //
    system_os_task(UserTask, USER_TASK_PRIORITY, userTaskQueue, USER_TASK_QUEUE_LENGTH);
}

Adding the above code to the basic application should generate data on GPIO0 and GPIO2 every second.

UDP Communication & WiFi Connectivity

The last part of the puzzle is to connect the board to the local WiFi network and to start to send data across the network.

WiFi Connection

For a basic WiFi connection where the router is providing a DHCP service then we need to let the firmware know the SSID of the network we wish to connect to and the password for the network:

const char ssid[32] = "--- Your SSID Here ---";
const char password[32] = "--- Your Password Here ---";
struct station_config stationConf;

wifi_set_opmode(STATION_MODE);
os_memcpy(&stationConf.ssid, ssid, 32);
os_memcpy(&stationConf.password, password, 32);
wifi_station_set_config(&stationConf);

Executing the above code will set the network parameters. The network connection is not established immediately but can take some time to become active. Network connectivity can be detected by the following code snippet:

struct ip_info info;

wifi_get_ip_info(STATION_IF, &info);
if (wifi_station_get_connect_status() != STATION_GOT_IP || info.ip.addr == 0)
{
    // Not connected yet !!!
}

UDP Communication

Network communication needs a connection structure to be set up. This holds information about the network connection type, IP addresses and ports. For this exercise we are using the UDP protocol and will broadcast the data to any UDP listener on the network. The UDP broadcast address is x.y.z.255, in this case 192.168.1.255. The code to set this up is as follows:

struct espconn *_ptrUDPServer;
uint8 udpServerIP[] = { 192, 168, 1, 255 };

//
//  Allocate an "espconn" for the UDP connection.
///
_ptrUDPServer = (struct espconn *) os_zalloc(sizeof(struct espconn));
_ptrUDPServer->type = ESPCONN_UDP;
_ptrUDPServer->state = ESPCONN_NONE;
_ptrUDPServer->proto.udp = (esp_udp *) os_zalloc(sizeof(esp_udp));
_ptrUDPServer->proto.udp->local_port = espconn_port();
_ptrUDPServer->proto.udp->remote_port = 11000;
os_memcpy(_ptrUDPServer->proto.udp->remote_ip, udpServerIP, 4);

The application can now start to send data over the network:

#define USER_DATA                   "ESP8266 - Data"

espconn_create(_ptrUDPServer);
espconn_sent(_ptrUDPServer, (uint8 *) USER_DATA, (uint16) strlen(USER_DATA));
espconn_delete(_ptrUDPServer);

The application should really perform some diagnostics in the above code in order to detect any errors and clean up where necessary.

Putting it all Together

A little rearrangement of the snippets above is necessary in order to put together a mode elegant solution, namely:

1. Output the results of function calls to the network to the GPIO pins
2. check for network connectivity before calling the network functions

Adding these modifications to the code gives the following final solution:

// ***************************************************************************
//
//  Basic UDP broadcast application for the ESP8266.
//
#include "at_custom.h"
#include "ets_sys.h"
#include "osapi.h"
#include "gpio.h"
#include "os_type.h"
#include "mem.h"
#include "user_config.h"
#include "ip_addr.h"
#include "espconn.h"
#include "user_interface.h"

#define USER_TASK_PRIORITY          0
#define USER_TASK_QUEUE_LENGTH      1
#define USER_DATA                   "ESP8266 - Data"
#define BB_DATA                     BIT0
#define BB_CLOCK                    BIT2

//
//  Callbacks for the OS task queue.
//
os_event_t userTaskQueue[USER_TASK_QUEUE_LENGTH];
struct espconn *_ptrUDPServer;
uint8 udpServerIP[] = { 192, 168, 1, 255 };

// ***************************************************************************
//
//  Forward declarations.
//
static void UserTask(os_event_t *events);

//
//  OS Timer structure holding information about the timer.
//
static volatile os_timer_t timerInformation;

//
//  Initialise the GPIO subsystem.
//
void InitialiseGPIO()
{
    gpio_init();
    //
    //  Set GPIO2 and GPIO0 to output mode.
    //
    PIN_FUNC_SELECT(PERIPHS_IO_MUX_GPIO2_U, FUNC_GPIO2);
    PIN_FUNC_SELECT(PERIPHS_IO_MUX_GPIO0_U, FUNC_GPIO0);
}

//
//  Bit bang the data out as serial (SPI-like) data.
//
void BitBang(uint8 byte)
{
    //
    //  Set the clock and data bits to be low.
    //
    gpio_output_set(0, BB_DATA, BB_DATA, 0);
    gpio_output_set(0, BB_CLOCK, BB_CLOCK, 0);
    //
    //  Output the data.
    //
    short bit;
    for (bit = 7; bit >= 0; bit--)
    {
        if (byte & (1 << bit))
        {
            gpio_output_set(BB_DATA, 0, BB_DATA, 0);
        }
        else
        {
            gpio_output_set(0, BB_DATA, BB_DATA, 0);
        }
        gpio_output_set(BB_CLOCK, 0, BB_CLOCK, 0);
        gpio_output_set(0, BB_CLOCK, BB_CLOCK, 0);
    }
    //
    //  Set the clock and data bits to be low.
    //
    gpio_output_set(0, BB_DATA, BB_DATA, 0);
    gpio_output_set(0, BB_CLOCK, BB_CLOCK, 0);
}

//
//  Timer function called periodically - try communicating with the world.
//
void TimerCallback(void *arg)
{
    struct ip_info info;

    wifi_get_ip_info(STATION_IF, &info);
    if (wifi_station_get_connect_status() != STATION_GOT_IP || info.ip.addr == 0)
    {
        BitBang(0x01);
    }
    else
    {
        BitBang(0x02);
        BitBang(espconn_create(_ptrUDPServer));
        BitBang(0x03);
        BitBang(espconn_sent(_ptrUDPServer, (uint8 *) USER_DATA, (uint16) strlen(USER_DATA)));
        BitBang(0x04);
        BitBang(espconn_delete(_ptrUDPServer));
        BitBang(0x05);
    }
}

//
//  User function which will be added to the task queue.
//
static void ICACHE_FLASH_ATTR UserTask(os_event_t *events)
{
    os_delay_us(10);
}

//
//  Initialise the application and add the user task to the task queue.
//
void ICACHE_FLASH_ATTR user_init()
{
    at_init();
    InitialiseGPIO();
    //
    //  Disarm timer.
    //
    os_timer_disarm(&timerInformation);
    //
    //  Start the network connection.
    //
    const char ssid[32] = "--- Your SSID Here ---";
    const char password[32] = "--- Your Password Here ---";
    struct station_config stationConf;
    
    wifi_set_opmode(STATION_MODE);
    os_memcpy(&stationConf.ssid, ssid, 32);
    os_memcpy(&stationConf.password, password, 32);
    wifi_station_set_config(&stationConf);
    //
    //  Allocate an "espconn" for the UDP connection.
    ///
    _ptrUDPServer = (struct espconn *) os_zalloc(sizeof(struct espconn));
    _ptrUDPServer->type = ESPCONN_UDP;
    _ptrUDPServer->state = ESPCONN_NONE;
    _ptrUDPServer->proto.udp = (esp_udp *) os_zalloc(sizeof(esp_udp));
    _ptrUDPServer->proto.udp->local_port = espconn_port();
    _ptrUDPServer->proto.udp->remote_port = 11000;
    os_memcpy(_ptrUDPServer->proto.udp->remote_ip, udpServerIP, 4);
    //
    //  Setup timer to call the timer function.
    //
    os_timer_setfn(&timerInformation, (os_timer_func_t *) TimerCallback, NULL);
    //
    //  Arm the timer:
    //      - timerInformation is the pointer to the OS Timer data structure.
    //      - 1000 is the timer duration in milliseconds.
    //      - 0 fire once and 1 for repeating
    //
    os_timer_arm(&timerInformation, 1000, 1);
    //
    //  Start OS tasks.
    //
    system_os_task(UserTask, USER_TASK_PRIORITY, userTaskQueue, USER_TASK_QUEUE_LENGTH);
}

Compiling and deploying this application to the ESP01 generates regular (1 second intervals) messages on the console application at the start of this post.

Conclusion

The journey with the ESP8266 has just started but it looks promising. The initial set up was not a simple as I would have liked as there are a number of parts all of which need to be brought together in order for the development cycle to start. Once these are all in place then coding is relatively easy even though there is little documentation. The community which is developing is certainly helping in this process.

On the whole this chipset offers an interesting opportunity especially with the like of Adafruit developing the Huzzah DIP friendly ESP12 board which has now received the appropriate certification for commercial WiFi use.

Having succeeded at getting a basic I2C master working on the STM8S it is not time to start to look at I2C slave devices on the STM8S.

The project will be a simple one, the STM8S will take a stream of bytes and perform simple addition. When requested, the STM8S will return the total as a 16-bit integer. Each new write request will clear the current total and start the whole process again.

Simple enough so let’s get started.

This post is a fairly long as it contains a lot of code so it might be a good time to grab a beer and settle down.

Simple Slave Adder

The slave adder is a simple device running over I2C. The devices has the following properties:

1. Acts as an I2C slave device with address 0x50
2. I2C bus speed is 50 KHz
3. Upon receiving a valid start condition and address for a write operation the device will clear the current total.
4. Bytes written to the device will be summed and a running total kept.
5. A start condition with a valid address for a read operation will return the current total as a 16-bit integer, MSB first.

A Netduino 3 will be used as the I2C bus master device. This runs the .NET Microframework and has an implementation of the I2C protocol built into the framework. This gives a good reference point for the master device i.e. someone else has debugged that so we can assume that the device is working as per the protocol specification.

As mentioned in the previous article on I2C Master devices, there is a really good article about the I2C protocol on Wikipedia. If you want more information about the protocol then I suggest you head over there.

Netduino Code

The initial version of the I2C master device will simply send two bytes of data for the I2C slave device to sum. The code is simple enough:

using Microsoft.SPOT;
using Microsoft.SPOT.Hardware;
using System.Threading;

namespace I2CMaster
{
    public class Program
    {
        public static void Main()
        {
            //
            //  Create a new I2C object on address 0x50 with the clock running at 50 KHz.
            //
            I2CDevice i2cBus = new I2CDevice(new I2CDevice.Configuration(0x50, 50));
            //
            //  Create a transaction to write two bytes of data to the I2C bus.
            //
            byte[] buffer = { 1, 2 };
            I2CDevice.I2CTransaction[] transactions = new I2CDevice.I2CTransaction[1];
            transactions[0] = I2CDevice.CreateWriteTransaction(buffer);
            while (true)
            {
                int bytesRead = i2cBus.Execute(transactions, 100);
                Thread.Sleep(1000);
            }
        }
    }
}

The above application creates a new instance of the I2CDevice with a device address of )x50 and a clock frequency of 50 KHz. A single transaction is created and the master writes the same two bytes to the I2C bus every second.

STM8S Code

As with the I2C Master article, much of the action happens in two places:

1. I2C device initialisation method
2. I2C Interrupt Service Routine (ISR)

The initialisation method sets up the I2C peripheral on the STM8S and enters the waiting state, waiting for the master to put data onto the I2C bus.

The ISR will deal with the actual data processing and in a full application it will also deal with any error conditions that may arise.

I2C Initialisation

The initialisation method sets up the I2C bus by performing the following tasks:

1. Setting the expected clock frequency
2. Setting the device address
3. Turning the interrupts on

The I2C peripheral must be disabled before configuring certain aspects of the peripheral, namely clock speeds:

I2C_CR1_PE = 0;

The peripheral needs to know the current master clock frequency, the I2C mode (Standard or Fast) and the clock divider values. These must all be configured whilst the peripheral is disabled.

I2C_FREQR = 16;                     //  Set the internal clock frequency (MHz).
I2C_CCRH_F_S = 0;                   //  I2C running is standard mode.
I2C_CCRL = 0xa0;                    //  SCL clock speed is 50 KHz.
I2C_CCRH_CCR = 0x00;

The device assumes that we will be using the standard 16 MHz clock speed which has been used in the other tutorials in this series. This is indicated by the I2C_FREQR register.

The values for the clock control register (I2C_CCRL and I2C_CCRH_CCR were simply taken from the STM8S programming reference manual. There is no coincidence that a bus speed of 50 KHz was chose, it is simply one of the reference values in the table. No point in creating work if you do not have to. If you want different speeds then you can either use one of the defined values in the remainder of the table or use the formulae provided.

The next step is to define the device address and addressing mode. I2C allows both 7 and 10 bit addressing modes. For simplicity this device will use a 7-bit device address:

I2C_OARH_ADDMODE = 0;               //  7 bit address mode.
I2C_OARH_ADD = 0;                   //  Set this device address to be 0x50.
I2C_OARL_ADD = 0x50;
I2C_OARH_ADDCONF = 1;               //  Docs say this must always be 1.

The device also allows for the maximum rise time to be configured. This example uses 17uS as the maximum time:

I2C_TRISER = 17;

The I2C peripheral allows for three different interrupt conditions to be defined, buffer interrupts, event interrupts (start condition etc.) and error interrupts. For convenience we will turn all of these on:

I2C_ITR_ITBUFEN = 1;                //  Buffer interrupt enabled.
I2C_ITR_ITEVTEN = 1;                //  Event interrupt enabled.
I2C_ITR_ITERREN = 1;

Now that the peripheral is configured we need to re-enable it:

I2C_CR1_PE = 1;

The last bit of initialisation is to configure the device to return an ACK after each byte:

I2C_CR2_ACK = 1;

At this point the I2C peripheral should be listening to the I2C bus for a start condition and the address 0x50.

I2C Interrupt Service Routine (ISR)

The ISR contains the code which processes the data and error conditions for the I2C peripheral. All of the I2C events share the same ISR and the ISR will need to interrogate the status registers in order to determine the exact reason for the interrupt.

Starting with an empty ISR we have the following code:

#pragma vector = I2C_RXNE_vector
__interrupt void I2C_IRQHandler()
{
}

There are several other vector names we could have chosen, they all contain the same value and the value I2C_RXNE_vector was chosen arbitrarily.

Assuming that there are no errors on the bus, the I2C master code above will cause the following events to be generated:

1. Address detection
2. Receive buffer not empty

This initial simple implementation can use these two events to clear the total when a new Start condition is received and add the current byte to the total when data is received.

if (I2C_SR1_ADDR)
{
    //
    //  In master mode, the address has been sent to the slave.
    //  Clear the status registers and wait for some data from the salve.
    //
    reg = I2C_SR1;
    reg = I2C_SR3;
    _total = 0;                 // New addition so clear the total.
    return;
}

I2C_SR1_ADDR should be set when an address is detected on the bus which matches the address currently in the address registers. As a starter application the code can simply assume that any address is going to be a write condition. The code can clear the totals and get ready to receive data. Note that this will be expanded later to take into consideration the fact that the master can perform both read and write operations.

The next event we need to consider is the receipt of data from the master. This will trigger the Receive Buffer Not Empty interrupt. This simple application should just read the buffer and add the current byte to the running total:

if (I2C_SR1_RXNE)
{
    //
    //  Received a new byte of data so add to the running total.
    //
    _total += I2C_DR;
    return;
}

As indicated earlier, the I2C ISR is a generic ISRT and is triggered for all I2C events. The initialisation code above has turned on the error interrupts as well as the data capture interrupts. Whilst none of the code in this article will handle error conditions, the ISR will output the status registers for diagnostic purposes:

PIN_ERROR = 1;
__no_operation();
PIN_ERROR = 0;
reg = I2C_SR1;
BitBang(reg);
reg = I2C_SR3;
BitBang(reg);

This will of course require suitable definitions and support methods.

Putting this all together gives an initial implementation of a slave device as:

//
//  This application demonstrates the principles behind developing an
//  I2C slave device on the STM8S microcontroller.  The application
//  will total the byte values written to it and then send the total
//  to the master when the device is read.
//
//  This software is provided under the CC BY-SA 3.0 licence.  A
//  copy of this licence can be found at:
//
//  http://creativecommons.org/licenses/by-sa/3.0/legalcode
//
#if defined DISCOVERY
    #include <iostm8S105c6.h>
#else
    #include <iostm8s103f3.h>
#endif
#include <intrinsics.h>

//
//  Define some pins to output diagnostic data.
//
#define PIN_BIT_BANG_DATA       PD_ODR_ODR4
#define PIN_BIT_BANG_CLOCK      PD_ODR_ODR5
#define PIN_ERROR               PD_ODR_ODR6

//
//  Somewhere to hold the sum.
//
int _total;

//
//  Bit bang data on the diagnostic pins.
//
void BitBang(unsigned char byte)
{
    for (short bit = 7; bit >= 0; bit--)
    {
        if (byte & (1 << bit))
        {
            PIN_BIT_BANG_DATA = 1;
        }
        else
        {
            PIN_BIT_BANG_DATA = 0;
        }
        PIN_BIT_BANG_CLOCK = 1;
        __no_operation();
        PIN_BIT_BANG_CLOCK = 0;
    }
    PIN_BIT_BANG_DATA = 0;
}

//
//  Set up the system clock to run at 16MHz using the internal oscillator.
//
void InitialiseSystemClock()
{
    CLK_ICKR = 0;                       //  Reset the Internal Clock Register.
    CLK_ICKR_HSIEN = 1;                 //  Enable the HSI.
    CLK_ECKR = 0;                       //  Disable the external clock.
    while (CLK_ICKR_HSIRDY == 0);       //  Wait for the HSI to be ready for use.
    CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.
    CLK_PCKENR1 = 0xff;                 //  Enable all peripheral clocks.
    CLK_PCKENR2 = 0xff;                 //  Ditto.
    CLK_CCOR = 0;                       //  Turn off CCO.
    CLK_HSITRIMR = 0;                   //  Turn off any HSIU trimming.
    CLK_SWIMCCR = 0;                    //  Set SWIM to run at clock / 2.
    CLK_SWR = 0xe1;                     //  Use HSI as the clock source.
    CLK_SWCR = 0;                       //  Reset the clock switch control register.
    CLK_SWCR_SWEN = 1;                  //  Enable switching.
    while (CLK_SWCR_SWBSY != 0);        //  Pause while the clock switch is busy.
}

//
//  Initialise the I2C system.
//
void InitialiseI2C()
{
    I2C_CR1_PE = 0;                     //  Disable I2C before configuration starts.
    //
    //  Set up the clock information.
    //
    I2C_FREQR = 16;                     //  Set the internal clock frequency (MHz).
    I2C_CCRH_F_S = 0;                   //  I2C running is standard mode.
    I2C_CCRL = 0xa0;                    //  SCL clock speed is 50 KHz.
    I2C_CCRH_CCR = 0x00;
    //
    //  Set the address of this device.
    //
    I2C_OARH_ADDMODE = 0;               //  7 bit address mode.
    I2C_OARH_ADD = 0;                   //  Set this device address to be 0x50.
    I2C_OARL_ADD = 0x50;
    I2C_OARH_ADDCONF = 1;               //  Docs say this must always be 1.
    //
    //  Set up the bus characteristics.
    //
    I2C_TRISER = 17;
    //
    //  Turn on the interrupts.
    //
    I2C_ITR_ITBUFEN = 1;                //  Buffer interrupt enabled.
    I2C_ITR_ITEVTEN = 1;                //  Event interrupt enabled.
    I2C_ITR_ITERREN = 1;
    //
    //  Configuration complete so turn the peripheral on.
    //
    I2C_CR1_PE = 1;
    //
    //  Acknowledge each byte with an ACK signal.
    //
    I2C_CR2_ACK = 1;
}

//
//  I2C interrupts all share the same handler.
//
#pragma vector = I2C_RXNE_vector
__interrupt void I2C_IRQHandler()
{
    unsigned char reg;

    if (I2C_SR1_ADDR)
    {
        //
        //  Clear the status registers and wait for some data from the salve.
        //
        reg = I2C_SR1;
        reg = I2C_SR3;
        _total = 0;                 // New addition so clear the total.
        return;
    }
    if (I2C_SR1_RXNE)
    {
        //
        //  Received a new byte of data so add to the running total.
        //
        _total += I2C_DR;
        return;
    }
    //
    //  Send a diagnostic signal to indicate we have cleared
    //  the error condition.
    //
    PIN_ERROR = 1;
    __no_operation();
    PIN_ERROR = 0;
    //
    //  If we get here then we have an error so clear
    //  the error, output the status registers and continue.
    //
    reg = I2C_SR1;
    BitBang(reg);
    reg = I2C_SR3;
    BitBang(reg);
}

//
//  Main program loop.
//
int main()
{
    _total = 0;
    __disable_interrupt();
    //
    //  Initialise Port D.
    //
    PD_ODR = 0;             //  All pins are turned off.
    PD_DDR_DDR4 = 1;        //  Port D, bit 4 is output.
    PD_CR1_C14 = 1;         //  Pin is set to Push-Pull mode.
    PD_CR2_C24 = 1;         //  Pin can run up to 10 MHz.
    //
    PD_DDR_DDR5 = 1;        //  Port D, bit 5 is output.
    PD_CR1_C15 = 1;         //  Pin is set to Push-Pull mode.
    PD_CR2_C25 = 1;         //  Pin can run up to 10 MHz.
    //
    PD_DDR_DDR6 = 1;        //  Port D, bit 6 is output.
    PD_CR1_C16 = 1;         //  Pin is set to Push-Pull mode.
    PD_CR2_C26 = 1;         //  Pin can run up to 10 MHz.
    //
    InitialiseSystemClock();
    InitialiseI2C();
    __enable_interrupt();
    while (1)
    {
        __wait_for_interrupt();
    }
}

Executing the Applications

At this stage a quick test with the two devices connected and the logic analyser will show the master device outputting data to the I2C bus. The correct operation of the I2C salve device can be verified by setting break points with in the two if statements in the ISR. Note that this will generate some errors but it is good enough to verify that the ISRs are being triggered correctly.

STM8 I2C Write Condition

The above shows the output from the Saleae Logic Analyser when the write transaction is sent to the bus by the Netduino.

Reading and Writing with I2C Slaves

At this point the above code should be accepting data from the Netduino 3. It is now time to expand the code to take into account the requirement to read back data from the slave device.

Netduino Code

The Netduino code will need to be modified to generate both a write transaction to send data to the I2C slave device and a read transaction to retrieve the results from the calculation.

using Microsoft.SPOT;
using Microsoft.SPOT.Hardware;
using System.Threading;

namespace I2CMaster
{
    public class Program
    {
        public static void Main()
        {
            //
            //  Create a new I2C object on address 0x50 with the clock running at 50 KHz.
            //
            I2CDevice i2cBus = new I2CDevice(new I2CDevice.Configuration(0x50, 50));
            //
            //  Create a transaction to write two bytes of data to the I2C bus.
            //
            byte[] buffer = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
            byte[] resultBuffer = new byte[2];
            I2CDevice.I2CTransaction[] transactions = new I2CDevice.I2CTransaction[2];
            transactions[0] = I2CDevice.CreateWriteTransaction(buffer);
            transactions[1] = I2CDevice.CreateReadTransaction(resultBuffer);
            while (true)
            {
                int bytesRead = i2cBus.Execute(transactions, 100);
                Thread.Sleep(1000);
            }
        }
    }
}

The transaction array has been expanded to contain a second transaction to read the data from the device. The data to be summed has also been expanded to include a further eight elements.

STM8S Code

A state machine will be used to allow the STM8S to work out what action should be taken within the ISR. This is required as the i”c peripheral will generate an event for the address detection for both the write transaction and the read transaction. The application will need to be able to differentiate between the two conditions as one requires the total to be cleared, the other requires that the total is sent back to the master device.

The state machine contains the following states:

1. Waiting for a write condition to start
2. Adding data to the running total
3. Sending the MSB of the total
4. Sending the LSB of the total

This is represented in code by an enum and a global variable:

typedef enum
{
    ISWaiting,                    //  Waiting for comms to start.
    ISAdding,                     //  Adding bytes of data.
    ISSendingMSB,                 //  Sending MSB of total.
    ISSendingLSB                  //  Sending LSB of total.
} I2CStateType;
I2CStateType _i2cState;

The ISR will need to be changed in order to deal with the state machine and also handle the slave transmitting data to the master device. The first change is to the address detection. This assumes that the main program loop initialises the state to ISWaiting:

if (I2C_SR1_ADDR)
{
    //
    //  Slave address received, work out if to expect a read or a write.
    //
    if (_i2cState == ISWaiting)
    {
        _i2cState = ISAdding;
        _total = 0;             // New addition so clear the total.
    }
    reg = I2C_SR1;
    reg = I2C_SR3;
    return;
}

The code above works out if the application is waiting for the first write condition (IsWaiting) or if the address detection has been triggered by a read condition (any other state).

Next, any other data reception is assumed to be data to be added to the total. The state is changed to IsAdding just in case:

if (I2C_SR1_RXNE)
{
    //
    //  Receiving data from the master so we must be adding.
    //
    _total += I2C_DR;
    _i2cState = ISAdding;
    return;
}

Next we have two new conditions we have not considered so far, the first is the fact that we need to transmit the result to the master. The request from the client will trigger a Transmit Buffer Empty event:

if (I2C_SR1_TXE)
{
    if (_i2cState == ISAdding)
    {
        I2C_DR = (_total >> 8) & 0xff;
        _i2cState = ISSendingMSB;
    }
    else
    {
        I2C_DR = _total & 0xff;
        _i2cState = ISSendingLSB;
    }
    return;
}

The next new condition is the acknowledge event. This is generated at the end of the master read operation. This will set the system back into a waiting condition:

if (I2C_SR2_AF)
{
    I2C_SR2_AF = 0;             //  End of slave transmission.
    _i2cState = ISWaiting;
    return;
}

The full application becomes:

//
//  This application demonstrates the principles behind developing an
//  I2C slave device on the STM8S microcontroller.  The application
//  will total the byte values written to it and then send the total
//  to the master when the device is read.
//
//  This software is provided under the CC BY-SA 3.0 licence.  A
//  copy of this licence can be found at:
//
//  http://creativecommons.org/licenses/by-sa/3.0/legalcode
//
#if defined DISCOVERY
    #include <iostm8S105c6.h>
#else
    #include <iostm8s103f3.h>
#endif
#include <intrinsics.h>

//
//  Define some pins to output diagnostic data.
//
#define PIN_BIT_BANG_DATA       PD_ODR_ODR4
#define PIN_BIT_BANG_CLOCK      PD_ODR_ODR5
#define PIN_ERROR               PD_ODR_ODR6

//
//  State machine for the I2C communications.
//
typedef enum
{
    ISWaiting,                    //  Waiting for comms to start.
    ISAdding,                     //  Adding bytes of data.
    ISSendingMSB,                 //  Sending MSB of total.
    ISSendingLSB                  //  Sending LSB of total.
} I2CStateType;
I2CStateType _i2cState;

//
//  Somewhere to hold the sum.
//
int _total;

//
//  Bit bang data on the diagnostic pins.
//
void BitBang(unsigned char byte)
{
    for (short bit = 7; bit >= 0; bit--)
    {
        if (byte & (1 << bit))
        {
            PIN_BIT_BANG_DATA = 1;
        }
        else
        {
            PIN_BIT_BANG_DATA = 0;
        }
        PIN_BIT_BANG_CLOCK = 1;
        __no_operation();
        PIN_BIT_BANG_CLOCK = 0;
    }
    PIN_BIT_BANG_DATA = 0;
}

//
//  Set up the system clock to run at 16MHz using the internal oscillator.
//
void InitialiseSystemClock()
{
    CLK_ICKR = 0;                       //  Reset the Internal Clock Register.
    CLK_ICKR_HSIEN = 1;                 //  Enable the HSI.
    CLK_ECKR = 0;                       //  Disable the external clock.
    while (CLK_ICKR_HSIRDY == 0);       //  Wait for the HSI to be ready for use.
    CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.
    CLK_PCKENR1 = 0xff;                 //  Enable all peripheral clocks.
    CLK_PCKENR2 = 0xff;                 //  Ditto.
    CLK_CCOR = 0;                       //  Turn off CCO.
    CLK_HSITRIMR = 0;                   //  Turn off any HSIU trimming.
    CLK_SWIMCCR = 0;                    //  Set SWIM to run at clock / 2.
    CLK_SWR = 0xe1;                     //  Use HSI as the clock source.
    CLK_SWCR = 0;                       //  Reset the clock switch control register.
    CLK_SWCR_SWEN = 1;                  //  Enable switching.
    while (CLK_SWCR_SWBSY != 0);        //  Pause while the clock switch is busy.
}

//
//  Set up Port D GPIO for diagnostics.
//
void InitialisePortD()
{
    PD_ODR = 0;             //  All pins are turned off.
    PD_DDR_DDR4 = 1;        //  Port D, bit 4 is output.
    PD_CR1_C14 = 1;         //  Pin is set to Push-Pull mode.
    PD_CR2_C24 = 1;         //  Pin can run up to 10 MHz.
    //
    PD_DDR_DDR5 = 1;        //  Port D, bit 5 is output.
    PD_CR1_C15 = 1;         //  Pin is set to Push-Pull mode.
    PD_CR2_C25 = 1;         //  Pin can run up to 10 MHz.
    //
    PD_DDR_DDR6 = 1;        //  Port D, bit 6 is output.
    PD_CR1_C16 = 1;         //  Pin is set to Push-Pull mode.
    PD_CR2_C26 = 1;         //  Pin can run up to 10 MHz.
}

//
//  Initialise the I2C system.
//
void InitialiseI2C()
{
    I2C_CR1_PE = 0;                     //  Disable I2C before configuration starts.
    //
    //  Set up the clock information.
    //
    I2C_FREQR = 16;                     //  Set the internal clock frequency (MHz).
    I2C_CCRH_F_S = 0;                   //  I2C running is standard mode.
    I2C_CCRL = 0xa0;                    //  SCL clock speed is 50 KHz.
    I2C_CCRH_CCR = 0x00;
    //
    //  Set the address of this device.
    //
    I2C_OARH_ADDMODE = 0;               //  7 bit address mode.
    I2C_OARH_ADD = 0;                   //  Set this device address to be 0x50.
    I2C_OARL_ADD = 0x50;
    I2C_OARH_ADDCONF = 1;               //  Docs say this must always be 1.
    //
    //  Set up the bus characteristics.
    //
    I2C_TRISER = 17;
    //
    //  Turn on the interrupts.
    //
    I2C_ITR_ITBUFEN = 1;                //  Buffer interrupt enabled.
    I2C_ITR_ITEVTEN = 1;                //  Event interrupt enabled.
    I2C_ITR_ITERREN = 1;
    //
    //  Configuration complete so turn the peripheral on.
    //
    I2C_CR1_PE = 1;
    //
    //  Set the acknowledge to be ACK.
    //
    I2C_CR2_ACK = 1;
}

//
//  I2C interrupts all share the same handler.
//
#pragma vector = I2C_RXNE_vector
__interrupt void I2C_IRQHandler()
{
    unsigned char reg;

    if (I2C_SR1_ADDR)
    {
        //
        //  Slave address received, work out if to expect a read or a write.
        //
        if (_i2cState == ISWaiting)
        {
            _i2cState = ISAdding;
            _total = 0;             // New addition so clear the total.
        }
        reg = I2C_SR1;
        reg = I2C_SR3;
        return;
    }
    if (I2C_SR1_RXNE)
    {
        //
        //  Receiving data from the master so we must be adding.
        //
        _total += I2C_DR;
        _i2cState = ISAdding;
        return;
    }
    if (I2C_SR1_TXE)
    {
        if (_i2cState == ISAdding)
        {
            I2C_DR = (_total >> 8) & 0xff;
            _i2cState = ISSendingMSB;
        }
        else
        {
            I2C_DR = _total & 0xff;
            _i2cState = ISSendingLSB;
        }
        return;
    }
    if (I2C_SR2_AF)
    {
        I2C_SR2_AF = 0;             //  End of slave transmission.
        _i2cState = ISWaiting;
        return;
    }
    //
    //  Send a diagnostic signal to indicate we have cleared
    //  the error condition.
    //
    PIN_ERROR = 1;
    __no_operation();
    PIN_ERROR = 0;
    //
    //  If we get here then we have an error so clear
    //  the error, output the status registers and continue.
    //
    reg = I2C_SR1;
    BitBang(reg);
    BitBang(I2C_SR2);
    reg = I2C_SR3;
    BitBang(reg);
}

//
//  Main program loop.
//
int main()
{
    _total = 0;
    _i2cState = ISWaiting;
    __disable_interrupt();
    InitialisePortD();
    InitialiseSystemClock();
    InitialiseI2C();
    __enable_interrupt();
    while (1)
    {
        __wait_for_interrupt();
    }
}

The above two pieces of code are fuller applications and if we execute these and hook up the logic anayser to the I2C bus we see the following output:

Adding 1 To 10

Reading From Multiple Devices

The I2C protocol allows for one or more devices with different slave addresses to be connected to the same I2C bus. The previous article used the TMP102 temperature sensor with slave address 0x48 and this article has created a slave device with address 0x50. It should therefore be possible to connect the two devices to the same bus and talk to each selectively.

Netduino Application

As with the previous examples, the Netduino 3 will be used as the I2C bus master. The applicaiton above will need to be merged with the code in the previous article.

using Microsoft.SPOT;
using Microsoft.SPOT.Hardware;
using System.Threading;

namespace I2CMaster
{
    public class Program
    {
        public static void Main()
        {
            //
            //  Create a new I2C object and the configurations for the STM8S and the TMP102.
            //
            I2CDevice.Configuration stm8s = new I2CDevice.Configuration(0x50, 50);
            I2CDevice.Configuration tmp102 = new I2CDevice.Configuration(0x48, 50);
            I2CDevice i2cBus = new I2CDevice(stm8s);
            //
            //  Create a transaction to write several bytes of data to the I2C bus.
            //
            byte[] buffer = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
            byte[] resultBuffer = new byte[2];
            I2CDevice.I2CTransaction[] transactions = new I2CDevice.I2CTransaction[2];
            transactions[0] = I2CDevice.CreateWriteTransaction(buffer);
            transactions[1] = I2CDevice.CreateReadTransaction(resultBuffer);
            //
            //  Create a transaction to read two bytes of data from the TMP102 sensor.
            //
            byte[] temperatureBuffer = new byte[2];
            I2CDevice.I2CTransaction[] reading = new I2CDevice.I2CTransaction[1];
            reading[0] = I2CDevice.CreateReadTransaction(temperatureBuffer);
            while (true)
            {
                //
                //  Read data from the I2C bus.
                //
                i2cBus.Config = stm8s;
                int bytesRead = i2cBus.Execute(transactions, 100);
                i2cBus.Config = tmp102;
                bytesRead = i2cBus.Execute(reading, 100);
                //
                //  Convert the reading into Centigrade and Fahrenheit.
                //
                int sensorReading = ((temperatureBuffer[0] << 8) | temperatureBuffer[1]) >> 4;
                double centigrade = sensorReading * 0.0625;
                double fahrenheit = centigrade * 1.8 + 32;
                //
                //  Display the readings in the debug window and pause before repeating.
                //
                Debug.Print(centigrade.ToString() + " C / " + fahrenheit.ToString() + " F");
                //
                //  Now display the results of the addition.
                //
                string message = "";
                for (int index = 0; index < buffer.Length; index++)
                {
                    message += buffer[index].ToString();
                    if (index == (buffer.Length - 1))
                    {
                        message += " = " + ((resultBuffer[0] * 256) + resultBuffer[1]).ToString();
                    }
                    else
                    {
                        message += " + ";
                    }
                }
                Debug.Print(message);
                Thread.Sleep(1000);
            }
        }
    }
}

The above code creates two I2C configuration object, one for the TMP102 and one for the STM8S device. The I2C bus object has the configuration changed depending upon which device is required.

Wiring up the Devices

Firstly, wire up the TMP102 sensor as described in the article on I2C master devices. The SDA and SCK lines should be connected to 3.3V via 4K7 resistors. Next connect the SDA and SCK lines from the STM8S device to the same point as SDA and SCK lines from the TMP102 sensor. Throw in the logic analyser connections and you get something like this:

TMP102 and STM8S I2C slave devices

Running the Application

Running the above two applications and starting the logic analyser generated the following output:

Reading From Two I2C Slaves

The write operation to the far left (W0x50) sends the 10 bytes to the STM8S device. This is followed by a read (R0x50) of two bytes from the same device, this can be seen about two thirds of the way from the left-hand side of the trace. The final operation is the read of the current temperature, R0x48, to the right of the trace.

Conclusion

The STM8S slave device above is a simple device. There are still plenty of modification which need to be made:

1. Add error handling
2. Allow for trapping conditions such as clock stretching

to name but a few. The code above should allow for simple devices to be put together is a short space of time.

Time for a small hardware update to the OpenIR project.

1. Removed the single AND gate from the output as the pulses are now controlled purely by software.
2. Repositioned the Power LED
3. Corrected a mistake with the positioning of the battery connections. These were Off by one
4. Added some additional markings to the board for clarification.

Over to the PCB manufacturer for new boards. In the meantime here are some images.

Schematic

OpenIR PCB Rev B Schematic

PCB Layout

OpenIR PCB Rev B

3d View

OpenIR PCB Rev B 3D Representation

A few nights ago I was working on implementing the processing the command sequences stored in the STM8S into IR pulses. The Win32 configuration application takes a series of high / low transitions measured in microseconds and turns the infra-red LED on and off accordingly. Doing this would require a signal capable of being triggered to the nearest microsecond.

This is where the plot fell apart.

Looking at the application now I think it is a case of over engineering the problem. By modifying the design parameters to be more realistic the problem can be overcome.

How Fast Do We Need To Be?

Previous articles have shown how a 2-3 MHz pulse can be generated by setting the system clock to 16MHz and toggling a port in a while loop. It has also been shown how Timer 2 can be used to generate a square wave using an interrupt based upon the clock pulse. The challenge is to generate and interrupt at a high enough frequency.

From the start it was recognised that the generation of a 1 MHz signal was ambitious using interrupts. Raising an interrupt on the STM8S takes 9 clock cycles and working on a 16MHz clock this only leaves 5 clock cycles for the work of the Interrupt SubRoutine (ISR) before the microcontroller returns to sleep and the whole cycle restarts.

So the question becomes, is this a problem?

Breaking out the calculator reveals that a 38KHz signal (a common frequency used for infra-red signals) has a period of 26 microseconds. So if we were to send a single pulse the ISR would have to respond within 26 microseconds. Having a carrier signal would not make sense if there were less than two carrier signal pulses within the infra-red signal. This would result in a minimum frequency of 19KHz which is well within the capabilities of the STM8S.

As I said, over engineering, or more likely, worrying about a problem which does not really exist.

Recreating the Nikon Shutter Trigger Signal

The changes which have been made are all aimed at making the remote control a universal remote control. It is i=now time to get back to the orignal aims of the project, namely to be able to trigger a Nikon camera.

The last post demonstrated the ability to create a configurable sequences of pulses / pauses and store these in the EEPROM. The next step is to convert these into a pulse sequence based upon a 38KHz carrier signal.

Although the predominant carrier frequency for an infra-red remote control is 38KHz, this implementation allows for the carrier frequency to be configured. In order to do this the software will need to change the counter and prescalar values of Timer 1, Channel 4. Four bytes of the EEPROM are reserved for these two values. The Windows application performs most of the work by calculating the required values:

int count = 0;
int prescalar = 0;
bool error = false;

if (value < 1)
{
    error = true;
}
else
{
    count = (int) (CLOCK_FREQUENCY / value);
    while ((prescalar < 65536) && (count > 65535))
    {
        prescalar++;
        count = (int) (CLOCK_FREQUENCY / (value * prescalar));
    }
    error = (prescalar == 65536);
}

if (error)
{
    float maxFrequency = CLOCK_FREQUENCY / 65535;
    throw new ArgumentOutOfRangeException("CarrierFrequency", string.Format("Carrier frequency should be between 1 and {0} Hz.", maxFrequency));
}
else
{
    _memory[PULSE_DURATION_OFFSET] = (byte) (count >> 8);
    _memory[PULSE_DURATION_OFFSET + 1] = (byte) (count & 0xff);
    _memory[PRESCALAR_OFFSET] = (byte) (prescalar >> 8);
    _memory[PRESCALAR_OFFSET + 1] = (byte) (prescalar & 0xff);
}

value is the desired frequency in Hz. The code cycles through the possible prescalar values until the right combination of counter and prescalar vales are obtained. The prescalar for Timer 1 is a value between 0 and 65535. One thing to note is that the value of the counter and the prescalar may not always return an exact frequency, for example, a carrier signal of 38KHz results in counter and prescalar values which generate a 38,004Hz signal. Not exact but given that the remote is using the internal oscillator we cannot expect an exact output anyway.

The perfectionist in me finds this to be disturbing but it is something I’ll have to live with.

Back on the remote control the application needs to use the configuration information in the EEPROM in order to regenerate the carrier signal for the remote control.

//
//  Set up Timer 1, channel 4 to output a PWM signal (the carrier signal).
//
void SetupTimer1()
{
    TIM1_ARRH = *((unsigned char *) EEPROM_PULSE_DURATION);
    TIM1_ARRL = *((unsigned char *) (EEPROM_PULSE_DURATION + 1));
    TIM1_PSCRH = *((unsigned char *) EEPROM_PRESCALAR_OFFSET);
    TIM1_PSCRL = *((unsigned char *) (EEPROM_PRESCALAR_OFFSET + 1));
    //
    //  Now configure Timer 1, channel 4.
    //
    TIM1_CCMR4_OC4M = 7;    //  Set up to use PWM mode 2.
    TIM1_CCER2_CC4E = 1;    //  Output is enabled.
    TIM1_CCER2_CC4P = 0;    //  Active is defined as high.
    //
    //  Work out the 50% duty cycle based upon the count.
    //
    unsigned short fiftyPercentDutyCycle;
    fiftyPercentDutyCycle = *((unsigned char *) EEPROM_PULSE_DURATION);
    fiftyPercentDutyCycle <<= 8;
    fiftyPercentDutyCycle += *((unsigned char *) (EEPROM_PULSE_DURATION + 1));
    fiftyPercentDutyCycle >>= 1;
    TIM1_CCR4H = ((fiftyPercentDutyCycle >> 8) & 0xff);
    TIM1_CCR4L = fiftyPercentDutyCycle & 0xff;
    //
    TIM1_BKR_MOE = 0;       //  Disable the main output.
    TIM1_CR1_CEN = 1;       //  Enable the timer.
}

Much of this code uses the counter values from the EEPROM to set up the PWM on Timer 1 Channel 4.

Two changes which have been made which do not impact the frequency of the carrier signal are the last two lines of the method. Previous versions of the code turned the time output on and the timer off. Here the Timer is turned on and the output off. This has been done to remove the need for the AND gate in the LED driver section of the circuit:

LED Driver With AND Gate

The AND gate takes the PWM signal from the STM8S along with an output enable signal (Port D, pin 3) from the STM8S. Changing the code to disable the timer removes the need for the output enable signal. The timer output is set to active high and when the timer is disabled the output pin of the timer goes into a high state. This problem is resolved by leaving the timer active and disabling the main output of the timer.

Another small change is to Timer 2, the timer used to determine the duration of the output of the carrier frequency. The Windows application takes the output and pause periods in microseconds. The Nikon shutter signal looks like this:

Nikon Pulse Sequence in Windows Application

When this is encoded in the EEPROM the sequence data looks as follows:

Nikon IR Sequence in EEPROM

The memory is encoded as follows:

Each of the commands is encoded as follows:

Now let’s look at how this data can be retrieved and used.

Timer 2 can be configured with a clock prescalar in the range 1 to 32,768. With a clock frequency of 16MHz a prescalar of 16 would drop the clock frequency to 1MHz. Doing this makes the calculation of the counter values as simple as recording the number of microseconds in the 16-bit values used for the counters. The setup code for Timer 2 becomes:

//
//  Setup Timer 2 ready to process the pulse data.
//
void SetupTimer2()
{
    TIM2_PSCR = 4;
    TIM2_ARRH = 0;
    TIM2_ARRL = 0;
    TIM2_EGR_UG = 1;
    TIM2_IER_UIE = 1;       //  Enable the update interrupts.
}

The Timer 2 interrupt handler currently looks are follows:

//
//  Timer 2 Overflow handler.
//
#pragma vector = TIM2_OVR_UIF_vector
__interrupt void TIM2_UPD_OVF_IRQHandler(void)
{
    _currentPulse++;
    if (_currentPulse == _numberOfPulses)
    {
        //
        //  We have processed the pulse data so stop now.
        //
        PD_ODR_ODR3 = 0;
        TIM2_CR1_CEN = 0;
        TIM1_BKR_MOE = 0;
        _currentState = STATE_WAITING_FOR_USER;
    }
    else
    {
        TIM2_ARRH = *_pulseDataAddress++;
        TIM2_ARRL = *_pulseDataAddress++;
        TIM1_BKR_MOE = !TIM1_BKR_MOE;       //  Toggle the T1 output.
        TIM2_CR1_URS = 1;
        TIM2_EGR_UG = 1;
    }
    //
    //  Reset the interrupt otherwise it will fire again straight away.
    //
    TIM2_SR1_UIF = 0;
}

Where _pulseDataAddress points to the next pulse to be processed and _numberOfPulses holds the count of the number of pulses in this sequence.

The code below walks through the list of commands looking for the pulse data for the sequence number held in the command variable.

//
//  We have enough command data.  Now work out where the command is.
//
_pulseDataAddress = (unsigned char *) (EEPROM_SEQUENCE_DATA_OFFSET);
unsigned char *length = (unsigned char *) EEPROM_SEQUENCE_LENGTH_TABLE_OFFSET;
for (unsigned char index = 0; index < command; index++)
{
    _pulseDataAddress += *((unsigned char *) length);
}

Now we have the pulse sequence is is necessary to set up Timer 2 and the global variables pointing to the pulse data and the number of pulses.

//
//  Now start processing the pulse data for the specified command.
//
_currentState = STATE_RUNNING;
_pulseDataAddress += MAX_PULSE_NAME_LENGTH;     //  Skip the name.
_numberOfPulses = *_pulseDataAddress++;
_currentPulse = 0;
TIM2_ARRH = *_pulseDataAddress++;
TIM2_ARRL = *_pulseDataAddress++;

The final step is to turn on Timer 1 output and start Timer 2 running:

//
//  Now we have everything ready we need to force the Timer 2 counters to
//  reload and enable Timers 1 & 2.
//
TIM2_CR1_URS = 1;
TIM2_EGR_UG = 1;
TIM1_BKR_MOE = 1;
TIM2_CR1_CEN = 1;

Calling TransmitPulseData should allow the command sequence to be output. Adding the line of code TransmitPulseData(0); after the configuration code and hooking up the logic analyser results in the following output:

Nikon IR Sequence in Logic Analyser

Conclusion

OpenIR has reached the point where command sequences can be edited and stored in the EEPROM and a single command sequence can be transmitted on a configurable carrier frequency.

Next steps:

1. The pulse sequence storage in the Windows application still needs to be tidied up.
2. When loading the EEPROM data the Windows application needs to regenerate the data for the list box containing the list of sequences.
3. Add commands to send a determined sequence / command number.
4. Redesign the board to remove the AND gate from the LED output.

Once the above has been completed additional connectivity options can be investigated.

For those who are interested, the source code for STM8S running the remote control is available for download.

Revision A of the board is now working and can send a single IR sequence out to a device in the real world when the on board switch is pressed. If OpenIR is to be truly universal the system needs to be able to send a multitude of commands not just a single command. In order to do this we need to be able to store IR command sequences and also allow the user to select which IR sequence is transmitted.

The STM8S has been set up to connect the TTL serial port to the FTDI and RedBear BLE board ports. Doing this allows communication with the outside world (PC, iPhone etc.). The proposed solution uses the serial TTL port to send commands to the STM8S and for the STM8S to store details of the IR signals (carrier frequency, active period etc.) in the on chip EEPROM.

The chip along with the chosen have a limit built into them, the fact that the free version of the IAR tools have an 8 KByte limit. This limits what can be achieved on the STM8S microcontroller.

Serial Commands

The STM8S will listen on the serial TTL port for commands from the outside world. The following list of commands are proposed as a starting point:

A close look at the above shows that commands 1, 3 and 4 are related as are commands 2, 4 and 6. They are either getting or setting blocks of memory in the STM8S EEPROM. Given the reduced memory available and the limits of the tools it may be optimal reduce this to reading and writing the contents of the EEPROM. The configuration data would be processed on a device with more memory (PC, iPhone etc.) and the EEPROM image built and transmitted to the STM8S. The STM8S then simply needs to update the EEPROM. The final command set becomes:

Layout of the EEPROM

The STM8S on the EEPROM stores the configuration of the remote control. The data stored is a mixture of basic configuration along details of the pulses for each command the remote control can transmit.

Conclusion

The basic layout of the EEPROM has been determined along with a proposed command sequence. The next step is to implement the STM8S code and some sample Windows code to configure the remote control.

September saw the end of an agonising few months (nearly a year in truth) working with a 32 x 32 LED matrix and smart LEDs (WS2811, Neopixels etc.). I’ve been trying to reliably control these LEDs with the STM32 Discovery board. It’s not that I cracked the problem with the STM32 but that I came across a cheap ready made solution to the problem, namely the Teensy 3.1. It has proved to be a painful reminder of something a software engineer should know, use libraries if you can. In the case of hardware prototyping the use of libraries also extends in part to the choice of hardware.

One of the main reasons I have resisted going down the route of using boards like the Teensy 3.1 is the Arduino IDE. I think that I have been spoiled by the richness of the Visual Studio IDE. Even IDES should as the IAR IDE for the STM8S leave something to be desired when you compare it to Visual Studio. This is the story of how I got the LEDs working and solved the IDE issue by using Visual Micro.

Problem Hardware

There are two pieces of hardware causing me problems at the moment:

1. 32 x 32 LED Matrix
2. 5mm Digital Addressable LEDs

These were purchased in the UK from Cool Components.

32 x 32 LED Matrix/Panel

The 32 x 32 LED matrix is often a component in larger displays.

LED Matrix

The displays are easily chained together although the power requirements quickly increase. Each panel can consume up to 2A depending upon the number of LEDs illuminated as any time.

These boards are designed to be controlled by FPGAs allowing for high frame rates. Sparkfun and Adafruit have tutorials on running these boards and they have found that you can run a single panel at 16MHz (i.e. you can run it on an Arduino but only just). Running two or more panels requires more power, both RAM and processor speed.

Digitally Addressable LEDs

The LEDs in question use the WS2811 constant current IC to drive RGB LEDs. These controllers use their own one-wire protocol (not to be confused with the Dallas one-wire protocol) to determine the PWM characteristics of the red, green and blue components of the light output. Over the years these controllers have shown up in LED strings, LED strips and more recently as the Adafruit Neopixel and now as individual through hole LEDs.

The main problem I have found with these LEDs, or more specifically the controllers, is the one-wire protocol. This requires very precise timing and this is often difficult to obtain on the hobbyist microcontrollers without a lot of very precise control.

Solving the problem

I decided to solve this problem using the Teensy 3.1. The thing which makes the Teensy 3.1 very attractive for this type of project is the add-on board, the SmartMatrix Shield. This board allows you to connect a Teensy directly on to the IDC headers of the LED Matrix.

In addition to the hardware, the Teensy 3.1 library contains ports of the equivalent Arduino libraries for the 32 x 32 LED matrix as well as LEDs controlled by the WS2811.

Teensy 3.1

The Teensy has been through several iterations, the Teensy 3.1 being the latest at the time of writing. The board is small, only 35mm x 18mm, and I was not really prepared for how small it actually is!

• 34 Digital I/O (3.3V but 5V tolerant)
• 21 analog inputs
• 12 PWM
• SPI, I2C, CAN bus
• ARM DSP extension

The foot print of the board allows it to be easily added to a breadboard.

The Teensy 3.1 board is also supplied with it’s own libraries in the form of Teensyduino. This is an add-on for the Arduino IDE and is compatible with many of the Arduino libraries. The Arduino compatible libraries provide the ability to work with the LED matrix and the addressable LEDs mentioned above as well as a wide variety of other devices.

Software

As I have stated earlier, I am not really a fan of the Arduino IDE. Do not misinterpret me, it represents a great piece of work, however the amount of time and money Microsoft have poured into Visual Studio over the years means that Visual Studio really does take some beating. This is where Visual Micro comes in.

Visual Micro is a Visual Studio 2008-2013 add-in which allows the development of Arduino sketches within the Visual Studio framework. This gives you all the power of Visual Studio (code refactoring etc.) while still allowing the development of Arduino sketches. The add-in is free until the end of 2014 with the option to purchase the debugger support for a nominal fee.

The add-in supports a good number of boards out of the box including the Teensy 3.1.

Setup is simple and the documentation is really good. There are a few additional steps for the Teensy 3.1 but this is cover in the Tips for Teensy page.

Once configured it was a simple task to create a new Teensy 3.1 project using the Neopixel sketch template. A little code reformatting gives the following code:

#include <Adafruit_NeoPixel.h>

//
//	Parameter 1 = number of pixels in strip
//	Parameter 2 = pin number (most are valid)
//	Parameter 3 = pixel type flags, add together as needed:
//		NEO_RGB     Pixels are wired for RGB bitstream
//		NEO_GRB     Pixels are wired for GRB bitstream
//		NEO_KHZ400  400 KHz bitstream (e.g. FLORA pixels)
//		NEO_KHZ800  800 KHz bitstream (e.g. High Density LED strip)
//
Adafruit_NeoPixel strip = Adafruit_NeoPixel(60, 6, NEO_RGB + NEO_KHZ800);

//
//	Input a value 0 to 255 to get a color value.
//	The colours are a transition r - g - b - back to r.
//
uint32_t Wheel(byte WheelPos)
{
	if (WheelPos < 85)
	{
		return strip.Color(WheelPos * 3, 255 - WheelPos * 3, 0);
	}
	else if (WheelPos < 170)
	{
		WheelPos -= 85;
		return strip.Color(255 - WheelPos * 3, 0, WheelPos * 3);
	}
	else
	{
		WheelPos -= 170;
		return strip.Color(0, WheelPos * 3, 255 - WheelPos * 3);
	}
}

//
//	Fill the dots one after the other with a colour
//
void colorWipe(uint32_t c, uint8_t wait)
{
	for (uint16_t i = 0; i < strip.numPixels(); i++)
	{
		strip.setPixelColor(i, c);
		strip.show();
		delay(wait);
	}
}

//
//  Set all LEDs to the same colour.
//
void rainbow(uint8_t wait)
{
	uint16_t i, j;

	for (j = 0; j < 256; j++)
	{
		for (i = 0; i < strip.numPixels(); i++)
		{
			strip.setPixelColor(i, Wheel((i + j) & 255));
		}
		strip.show();
		delay(wait);
	}
}

//
//	Slightly different, this makes the rainbow equally distributed throughout
//
void rainbowCycle(uint8_t wait)
{
	uint16_t i, j;

	for (j = 0; j < 256 * 5; j++)
	{ // 5 cycles of all colors on wheel
		for (i = 0; i < strip.numPixels(); i++)
		{
			strip.setPixelColor(i, Wheel(((i * 256 / strip.numPixels()) + j) & 255));
		}
		strip.show();
		delay(wait);
	}
}

//
//	Executed once at startup to set up the board.
//
void setup()
{
	strip.begin();
	strip.show(); // Initialize all pixels to 'off'
}

//
//	The actual main program loop.
//
void loop()
{
	rainbow(20);
	rainbowCycle(20);
}

This code creates an Adafruit_NeoPixel object with the pixels connected to pin 6 of the Teensy 3.1.

Connecting some LEDs top the Teensy and uploading the above code results in the following:

Modifying the code to add a counter will allow testing of the debugging features. The loop code is changed to have a counter variable to test the conditional breakpoint feature:

int count = 0;

//
//	The actual main program loop.
//
void loop()
{
	count++;
	digitalWrite(LED_PIN, HIGH);
	delay(500);
	digitalWrite(LED_PIN, LOW);
	delay(500);
	// Some example procedures showing how to display to the pixels:
	rainbow(20);
	rainbowCycle(20);
}

Adding a breakpoint after the counter increment and making the breakpoint conditional results in the following display:

Sample Code With Breakpoint

Nothing too strange here, pretty much standard for Visual Studio. The breakpoint window shows the breakpoint and the condition:

Debugger Breakpoint Window

Running the code brings up the debugger expression window:

Debugger Expression Window

The one thing which experienced users of Visual Studio will find unfamiliar is the way in which single stepping works. Unlike say Win32 development where single stepping takes you to the next line of code and then waits, with Visual Micro, single stepping runs the code to the next breakpoint.

Conclusion

Visual Studio is without doubt one of the best software development environments available. The addition of Visual Micro brings the power of Visual Studio to the Arduino world. Whilst the debugging features (single stepping) may a little unfamiliar to Visual Studio users they are not so strange as to be unusable.

Both the Teensy 3.1 and Visual Micro are a welcome addition to the development tools and it is highly likely that they will appear in future posts.

A few weeks ago I acquired and Electric Imp as I was interested in how this could be used to prototype and connect a device to the Internet. Such a device would become part of the Internet of Things.

In order to investigate the possibility of using this device I decided to monitor the temperate of my office and log the data on the Internet.

Electric Imp

Electric Imp offers a starting point for hardware and software engineers wishing to develop for the Internet of Things. The simplest format for prototyping is probably the Imp001 and a breakout board. The Imp001 is an Electric Imp in SD format. The card is fully FCC certified and so offers a simple way of using a wireless network to connect a project to the Internet. The SD card contains a Cortex-M3 processor, 802.11b/g/n transceiver and antenna and once connected to a WiFi network with internet connectivity can be programmed using the cloud based development environment.

The Electric Imp is connected to a WiFi network using an application called BlinkUp. This application is a free download for iPhone and Android. The phone application takes details about the local WiFi network (name, password etc.) and sends this to the Imp001 by blinking the screen (hence the name BlinkUp) whilst the phone is held against the LED on the SD card.

Software for the Electric Imp is developed using the online IDE provided by Electric Imp. The development environment offers the ability to develop code which runs on the Electric Imp (Device code) itself as well as a component which runs in the cloud (Agent code). A user account is setup by following the link to the Log in page from the Electric Imp web site.

Sparkfun Data Logging Service

Sparkfun have recently started to provide a cloud based data logging service which is free to use for a limited amount of data. The system uses a circular 50 MByte data store for each data stream. Sign up is simple, just follow the Create link from the main page. The data streams can be both public and private. If the amount of data storage required is greater than 50MB or the application requires a greater level of privacy then the source is freely available for download by following the DFeploy link from the main page.

Once created, the data stream is accessed using public and private keys, the private keys allow data to be written to the data stream. The public key allows the public data stream to be viewed or data to be retrieved for use in say charting.

Temperature Logging

The principles involved in linking the local hardware to the Sparkfun cloud server will be illustrated by logging the local temperature using a temperature sensor and then sending the data to Sparkfun’s servers. The data will be retrieved and displayed on a web page.

Hardware

The bill of materials (BOM) for this project is as follows:

1. Electric Imp (Imp001)
2. Electric Imp Breakout board
3. LM35 Temperature Sensor
4. LED and current limiting resistor
5. Connectors
6. Breadboard and miscellaneous wire
7. USB cable and power supply (your computer can act as a power supply if necessary)

Solder the connectors to the breakout board and insert the connectors into the breadboard.

Next, connect the current limiting resistor to Pin 9 on the breakout board and the LED. Connect the other leg of the LED through to ground.

Finally connect the LM35 temperature sensor to V_cc, Ground and the sensor output to Pin 2 of the breakout board.

Follow the instructions on the Electric Imp web site for downloading the BlinkUp software and configuring the Electric Imp. Configure the Imp and connect to the local WiFi network.

Electric Imp Software

The Electric Imp software is split into two components:

1. Device code running on the Electric Imp hardware
2. Agent code which runs on the Electric Imp cloud servers

The first step is to register with the Electric Imp web site for a developer account. Once completed you will be presented with the developer IDE.

To record the temperature the Electric Imp will record the temperature every 5 seconds. These readings will be summed and averaged over a one minute period. The average will sent to the Electric Imp servers, the average cleared and the whole process will restart. This will provide a continuous stream of temperature readings while the Imp is powered.

The Electric Imp servers will run Agent code which will listen for data/commands from the Electric Imp device code. The Agent on the server will then post the data to the Sparkfun server.

The code for the Device and the Agent is written in a C like language called Squirrel.

Device Code

The analog port on the Electric Imp returns a value in the range 0 to 65,535. The maximum value represents a voltage of 3.3V. The temperature sensor selected outputs 10mv per degree C. The maximum range of the values for this sensor is 0V to 1.55V given the operating range for the LM35.

The LED has been added to demonstrate when the board is taking a temperature reading. The LED will flash each time a reading is taken.

Firstly, some space in needed for the supporting variables:

//
//  Create a global variable to allow control of the LED.
//
led <- hardware.pin9;
//
//  Create a global variable for the temperature sensor.
//
temperatureSensor <- hardware.pin2;
// 
//  Configure led to be a digital output.
//
led.configure(DIGITAL_OUT);
//
//  Configure the temperature sensor to be an analog input.
//
temperatureSensor.configure(ANALOG_IN);
//
//  This name will be sent to the stream each update:
//
local impName = "Imp%20" + imp.getmacaddress();
//
//  Variables related to averaging the temperature.
//
readingCount <- 0;
readingSum <- 0.0;
// 
//  Create a global variables for the sensor readings.
//
temperature <- 0.0;
sensorValue <- 0;

Next, the main application loop (function), the first thing this should do is to turn on the LED to show that the application is active:

//
//  Main program loop.
//
function main()
{
    led.write(1);

Next, take the temperature sensor reading and convert to centigrade and add to the ongoing sum:

    sensorValue = temperatureSensor.read();
    temperature = ((sensorValue * 3.3) / 65535) * 100;
    readingCount++;
    readingSum += temperature;

Next, check if the number of readings has reached 12 (60 seconds). If we have 12 readings then take the average and send this to the Electric Imp Agent:

    if (readingCount == 12)
    {
        local average = readingSum / readingCount;
        server.log("Average temperature = " + average);
        local data = "";
        data = "Temperature=" + average;
        agent.send("postData", data);
        readingCount = 0;
        readingSum = 0.0;
    }

The server.log statement sends the logging information to the servers. This is not used anywhere, simply logged. The data is sent to the Agent in the agent.send(“postData”, data) statement.

Next, pause and then turn the LED off:

    imp.sleep(0.5);
    led.write(0);

The whole process should be repeated 4.5 seconds later (remember there is a 0.5 seconds pause above) to take readings every 5 seconds.

    //
    //  Schedule imp to wakeup and repeat.
    //
    imp.wakeup(4.5, main);
}

Finally the main loop should be executed:

//
//  Start the main program loop.
//
main();

Agent Code

The Agent code is responsible for listening for data from the device. The code used is actually provided by Sparkfun and is produced here more or less unaltered:

/*****************************************************************
Phant Imp (Agent)
Post data to SparkFun's data stream server system (phant) using
an Electric Imp
Jim Lindblom @ SparkFun Electronics
Original Creation Date: July 7, 2014

Description

Before uploading this sketch, there are a number of vars that need adjusting:
1. Phant Stuff: Fill in your data stream's public, private, and 
data keys before uploading!

This code is beerware; if you see me (or any other SparkFun 
employee) at the local, and you've found our code helpful, please 
buy us a round!

Distributed as-is; no warranty is given.
*****************************************************************/

//
//  Phant Stuff configuration information.
//
local publicKey = "Your-Public-Key-Goes-Here";      // Your Phant public key
local privateKey = "Your-Private-Key-Goes-Here";    // Your Phant private key
local phantServer = "data.sparkfun.com";            // Your Phant server, base URL, no HTTP

//
//  When the agent receives a "postData" string from the device, use the
//  dataString string to construct a HTTP POST, and send it to the server.
//
device.on("postData", function(dataString)
    {
        server.log("Sending " + dataString); // Print a debug message
        //
        //  Construct the base URL: https://data.sparkfun.com/input/PUBLIC_KEY:
        //
        local phantURL = "https://" +  phantServer + "/input/" + publicKey;
        //
        //  Construct the headers: e.g. "Phant-Private-Key: PRIVATE_KEY"
        //
        local phantHeaders = {"Phant-Private-Key": privateKey, "connection": "close"};
        //
        //  Send the POST to phantURL, with phantHeaders, and dataString data.
        //
        local request = http.post(phantURL, phantHeaders, dataString);
        //
        //  Get the response from the server, and send it out the debug window:
        //
        local response = request.sendsync();
        server.log("Phant response: " + response.body);
    }
);

The code is clearly commented and self explanatory.

Results

Building the above in the IDE should deploy the device code to the Electric Imp should result in the temperature being collected by the device, sent to the Electric Imp server and then from there on to Sparkfun’s servers. The data can be viewed in it’s raw form by browsing to your data stream using a URL such as: https://data.sparkfun.com/streams/Your-Public-Key-Here. This URL will have been supplied on the account creation page for the Sparkfun data service.

The data can also be retrieved using Javascript:

<!DOCTYPE html>
<html>
  <head>
    <!-- EXTERNAL LIBS-->
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
    <script src="https://www.google.com/jsapi"></script>

    <!-- EXAMPLE SCRIPT -->
    <script>

      // onload callback
      function drawChart() {

        var public_key = 'Your-Public-Key-Here';

        // JSONP request
        var jsonData = $.ajax({
          url: 'https://data.sparkfun.com/output/' + public_key + '.json',
          data: {page: 1},
          dataType: 'jsonp',
        }).done(function (results) {

          var data = new google.visualization.DataTable();

          data.addColumn('datetime', 'Time');
          data.addColumn('number', 'Temperature');

          $.each(results, function (i, row) {
            data.addRow([
              (new Date(row.timestamp)),
              parseFloat(row.Temperature),
            ]);
          });

          var chart = new google.visualization.LineChart($('#chart').get(0));

          chart.draw(data, {
            title: 'Room Temperature', height: 500, is3D: true
          });

        });

      }

      // load chart lib
      google.load('visualization', '1', {
        packages: ['corechart']
      });

      // call drawChart once google charts is loaded
      google.setOnLoadCallback(drawChart);

    </script>
  </head>
  <body>
    <div id="chart" style="width: 100%;"></div>
  </body>
</html>

The above uses Google’s chart API to generate a chart from the data stored in the Sparkfun servers (thanks for Sparkfun for the code). Save the above page to a web server and browse to the page and you will see something like the following:

Room Temperature Chart

The chart shows the fall and rise in temperature in a room over a period of 48 hours.

Conclusion

The Electric Imp offers a simple method for connecting a device to the Electric Imp servers on the Internet. The Agent code can then pass this data on to services provided by additional third parties.

The device options used here would increase the cost of any device produced but as a proof of concept they offer a simple and convenient way of demonstrating how a device can interact with the outside world over the Internet. This example took less than 3 hours to research, build and complete – good going considering how complex this would be if this were completed in a conventional manner (Arduino, WiFly shield etc.).

Yesterday I received my Intel Galileo rev 1 board. I know the rev 2 board is available but recently the Windows on Devices program have release the necessary firmware etc to upgrade a Galileo rev 1 board to enable it to run Windows.

See the end of this article for an update added on 6th Sept 2014.

Upgrading the Firmware

The first step was to check athe firmware and upgrade it if necessary. In my case it was necessary. Intel provide a comprehensive set of instructions on how to do this. The upgrade process took about 10 minutes.

Creating a Windows SD Card

The next step is to write Windows to a micro SD card. This step of the process took the longest to complete, about 25-30 minutes.

Booting to Windows

The next step is to verify that Windows has loaded correctly. Insert the card, power on and then waiting for 2 minutes for Windows to boot. If successful you should be able to telnet to the device. I used PTTYPortable to do this and was presented with the login request.

Testing the Board

One of the first tests I normally perform is to deploy a Blinky application to the board. Once I am happy that I can deploy applications to the board I speed up Blinky by removing any code which would cause a pause. The result should be an indicator of the performance of the board and the software. So let’s give it a go.

The Galileo board offers two methods for deploying Blinky to the board:

1. Arduino UI
2. Visual Studio Windows application

The first method uses the board without the Windows SD card image, the second deploys a Windows application to the SD card and runs as a Windows application.

Our starting point for the tests will be the standard Blink application:

/*
  Blink
  Turns on an LED on for one second, then off for one second, repeatedly.
 
  This example code is in the public domain.
*/
 
// Pin 13 has an LED connected on most Arduino boards.
// give it a name:
int led = 13;

// the setup routine runs once when you press reset:
void setup()
{                
    // initialize the digital pin as an output.
    pinMode(led, OUTPUT);     
}

// the loop routine runs over and over again forever:
void loop()
{
    digitalWrite(led, HIGH);   // turn the LED on (HIGH is the voltage level)
    delay(1000);               // wait for a second
    digitalWrite(led, LOW);    // turn the LED off by making the voltage LOW
    delay(1000);               // wait for a second
}

Arduino UI

Using this application in the Arduino UI for the Galileo is simple. In fact the application is one of the samples (File -> Examples -> 01.Basics -> Blink). Loading this sketch and deploying to the Galileo starts the on board LED blinking at a steadily at 1 Hz.

Visual Studio Windows Application

Microsoft have provided a Wiring API for use in Visual Studio. This allows access to the “Arduino” hardware from a Windows application. An equivalent Windows version of the same application is:

#include "stdafx.h"
#include "arduino.h"

int _tmain(int argc, _TCHAR* argv[])
{
    return RunArduinoSketch();
}

int led = 13;  // This is the pin the LED is attached to.

void setup()
{
    pinMode(led, OUTPUT); // Configure the pin for OUTPUT so you can turn on the LED.
}

// the loop routine runs over and over again forever:
void loop()
{
    digitalWrite(led, LOW);    // turn the LED off by making the voltage LOW
    Log(L"LED OFF\n");
    delay(1000);               // wait for a second
    digitalWrite(led, HIGH);    // turn the LED on by making the voltage HIGH
    Log(L"LED ON\n");
    delay(1000);               // wait for a second
}

Much of the application looks the same as the Arduino application. The main differences are the addition of the _tmain method and the Log statement. The _tmain method acts as the entry point for the application and provides a method for running an Arduino sketch (as above) or some other program logic.

The Log statements generate debug information which is displayed in Visual Studio’s Output window.

Deploying this application to the board results in… NOTHING!

Digging a little deeper into the examples section of the web site reveals the On Board LED example:

// Main.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "arduino.h"

int _tmain(int argc, _TCHAR* argv[])
{
    return RunArduinoSketch();
}

//This application flashes the on board LED of the Galileo board by calling GPIO functions directly in the embprpusr.dll instead of using the Arduino layer.

ULONG state = LOW; // keeps track of the state of the on-board LED

void setup()
{
    GpioSetDir(LED_BUILTIN, OUTPUT); // Sets the pin to output
}

void loop()
{
    if (HIGH == state)
    {
	    state = LOW;
	} 
    else
    {
	    state = HIGH;
	}
    GpioWrite(LED_BUILTIN, state); // Writes to the pin, setting its value either HIGH (on) or LOW (off)
    Log(L"LED %s\n", (HIGH == state ? L"ON" : L"OFF"));
    Sleep(1000);
}

Compiling and deploying this application to the board results in the steady flashing of the on board LED.

Reverting to the previous sample and hooking up an oscilloscope reveals that pin 13 is indeed being toggled at 1 Hz it is just not connected directly to the on board LED.

How Fast Can We Go?

Now to find out how fast the board will actually run. Starting with the Arduino example, remove the delay statements, recompile and deploy to the board. Doing this resulted in a square wave with a 50% duty cycle and a frequency of 221Hz. That’s right Hz, not KHz!

Removing the delay and logging statements and deploying the Windows application results in a square wave. This time a 60Hz square wave with a 30% duty cycle is displayed on the oscilloscope.

There must be something wrong. Surely this board with a 400MHz processor should run faster than this.

What About the Netduino?

The Netduino has always had one issue when compared with Arduino and other similar board. Namely it is running interpreted code which is not real time due to the nature of the .NET Microframework and the way the framework runs. I have performed similar tests and I was convinced that it was faster. Only one way to find out, deploy some code to the Netduino Plus 2. This board runs the .NET Microframework on the STM32 family of microcontrollers at 168 MHz. The equivalent code to the two examples above is:

using Microsoft.SPOT.Hardware;
using SecretLabs.NETMF.Hardware.Netduino;

namespace NetduinoPlus2
{
    public class Program
    {
        public static void Main()
        {
            OutputPort dp = new OutputPort(Pins.GPIO_PIN_D13, false);
            while (true)
            {
                dp.Write(!dp.Read());
            }
        }
    }
}

Deploying this to the Netduino PLus 2 and hooking up the oscilloscope results in a square wave with a 50% duty cycle at 17.8 KHz.

Much faster than the Galileo board.

Update 6th Sept 2014

I have been looking through the schematic for the Galileo Rev 1 board and found that not all of the GPIO pins are connected through the CY8C9540A chip but are in fact connected directly top the Quark processor. These GPIOs should be capable of higher speeds. A quick test shows that these pins (Digital 2, 3 & 10) can all generate a 1.16 KHz square wave for an application compiled in debug mode. Compiling the same applications in release more and running the application on the Galileo increases the frequency from 1.16 KHz to 1.27 KHz.

Conclusion

I’ve only had this board for a few hours but I have deployed a few of the examples. The raw GPIO speed appears lower than the interpreted .NET Microframework equivalent. The Galileo has access to a network port with easy access to the Arduino Wiring API but then so does the .NET Microframework.

Some further investigation is required I believe.

Window watchdogs provide a mechanism for detecting software failures in two ways, firstly an early reset of the watchdog and secondly a failure to reset the watchdog in time. In this post we investigate how the window watchdog can be use and illustrate with some examples.

Hardware

Window Watchdog Control Register – WWDG_CR

This register has two components, the timer counter and the enable bit (WWDG_CR_WDGA – see below). The microcontroller will be reset when one of two possible conditions:

• The counter switches from 0x40 to 0x3f (i.e. bit 6 in the counter changes from 1 to 0)
• The counter is reset when the counter value is greater than the watchdog window register

Writing 0 to bit 6 will cause the microcontroller to be reset immediately.

Assuming that WWDG_WR contains the default reset value then the time out period (in milliseconds) is defined as follows:

t_WWDG = t_CPU * 12288 * (WWDG_CR & 0x3f)

where t_CPU = 1 / f_master

On the STM8S running at 16MHz a value of 0x40 represents one count which is equal to 0.768ms. So at 16MHz the time out period is:

t_WWDG = 0.768 * (WWDG_CR & 0x3f)

Window Watchdog Enable Register – WWDG_CR_WDGA

Switch the Window Watchdog on (set to 1) or off (set to 0).

Window Watchdog Window Register – WWDG_WR

This register defines a time period where the watchdog counter should not be reset. If the counter (WWDG_CR) is reset when the counter value is greater than the value in this register the microcontroller will be reset. This can be illustrated as follows:

Watchdog Sequence

We can calculate the value of t_WindowStart and t_timeout as follows (assuming a 16MHz clock):

t_WindowStart = 0.768 * ((WWDG_CR_initial & 0x3f) – WWDG_WR)

and

t_timeout = 0.768 * (WWDG_CR & 0x3f)

where WWDG_CR_initial is the initial value in the WWDG_CR register.

The default reset value for this register is 0x7f which means that the counter can be reset at any time. In this case, a reset will only be generated if the counter drops below 0x40.

One important point to note is that when the window register is used the value written to the counter (WWDG_CR) must be between 0xc0 and 0x7f. This causes the counter to be reset and the counter value to be reset simultaneously.

Software

The function of the Window Watchdog will be illustrated using the following three examples:

• WWDG_CR not reset
• WWDG_CR reset outside the reset window
• WWDG_CR reset inside the reset window

The first thing we need to do is add some code which will be used in all of the examples.

Common Code

Firstly, lets add the code which will be common to all of the examples:

//
//  This program demonstrates how to use the Window Watchdog on the STM8S
//  microcontroller.
//
//  This software is provided under the CC BY-SA 3.0 licence.  A
//  copy of this licence can be found at:
//
//  http://creativecommons.org/licenses/by-sa/3.0/legalcode
//
#include <iostm8S105c6.h>
#include <intrinsics.h>

//--------------------------------------------------------------------------------
//
//  Setup the system clock to run at 16MHz using the internal oscillator.
//
void InitialiseSystemClock()
{
    CLK_ICKR = 0;                       //  Reset the Internal Clock Register.
    CLK_ICKR_HSIEN = 1;                 //  Enable the HSI.
    CLK_ECKR = 0;                       //  Disable the external clock.
    while (CLK_ICKR_HSIRDY == 0);       //  Wait for the HSI to be ready for use.
    CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.
    CLK_PCKENR1 = 0xff;                 //  Enable all peripheral clocks.
    CLK_PCKENR2 = 0xff;                 //  Ditto.
    CLK_CCOR = 0;                       //  Turn off CCO.
    CLK_HSITRIMR = 0;                   //  Turn off any HSIU trimming.
    CLK_SWIMCCR = 0;                    //  Set SWIM to run at clock / 2.
    CLK_SWR = 0xe1;                     //  Use HSI as the clock source.
    CLK_SWCR = 0;                       //  Reset the clock switch control register.
    CLK_SWCR_SWEN = 1;                  //  Enable switching.
    while (CLK_SWCR_SWBSY != 0);        //  Pause while the clock switch is busy.
}

//--------------------------------------------------------------------------------
//
//  Initialise the ports.
//
//  Configure all of Port D for output.
//
void InitialisePorts()
{
    PD_ODR = 0;             //  All pins are turned off.
    PD_DDR = 0xff;          //  All pins are outputs.
    PD_CR1 = 0xff;          //  Push-Pull outputs.
    PD_CR2 = 0xff;          //  Output speeds up to 10 MHz.
}

This code has been used many times in The Way of the Register series of posts. It simply sets the system clock to the high speed internal clock and configures Port D for output.

Example 1 – Continuous Reset

This example sets the Windows Watchdog running and then waits for the watchdog to trigger the system reset. We indicate that the application is running by generating a pulse on Port D, pin 2.

//--------------------------------------------------------------------------------
//
//  Initialise the Windows Watchdog.
//
void InitialiseWWDG()
{
    PD_ODR_ODR2 = 1;
    __no_operation();
    __no_operation();
    __no_operation();
    __no_operation();
    PD_ODR_ODR2 = 0;
    WWDG_CR = 0xc0;
}

The __no_operation() instruction in the above code allow the pulse to stabilise on the pin.

The WWDG_CR is set to 0xc0 to both set the value in the counter and enable the watchdog at the same time. This sets bit 6 in the counter to 11 and the remaining bits to 0 (i.e. the counter is set to 0x40). The effect of this is that the first down count event will cause bit 6 to be cleared and the counter to be set to 0x3f. This will trigger the reset event.

The main program simply sets everything up and then “pauses” by simply waiting for an interrupt:

//--------------------------------------------------------------------------------
//
//  Main program loop.
//
int main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    InitialisePorts();
    InitialiseWWDG();
    __enable_interrupt();
    //
    //  Main program loop.
    //
    while (1)
    {
        __wait_for_interrupt();
    }
}

If we run this application and connect PD2 and NRST to the oscilloscope we see the following trace:

Initial continuous reset

The yellow trace shows the reset line being pulled low and gradually returning to high. The drop shows where the watchdog has caused the reset pin to be pulled low and the microcontroller to be reset. The blue trace shows the pulse on PD2. If we measure the time difference between the pulse on PD2 and the time that the reset pin is pulled low we find that this is 770uS. This is very close to the time for one count, 768uS.

Continuous reset

Measuring the time difference we get a value of 6.9mS.

Example 2 – Reset Outside Watchdog Window

Using the above code as a starting point we will look at the effects of the watch dog window register (WWDG_WR). This defines when the application is allowed to change the value in the counter. The first task is to change the initial value in the control register to 0xc1 and verify that we get a rest every 1.54ms (2 x timer period). So change the InitialiseWWDG method to the following:

void InitialiseWWDG()
{
    PD_ODR_ODR2 = 1;
    __no_operation();
    __no_operation();
    __no_operation();
    __no_operation();
    PD_ODR_ODR2 = 0;
    WWDG_CR = 0xc1;
}

Running this application on the STM8S Discovery board results in the following traces:

Reset Outside window

Now we have two counts (1.54mS) in order to change the value in the control register. First task is to modify the InitialiseWWDG method to define the window. We will define this to be 0x40:

void InitialiseWWDG()
{
    PD_ODR_ODR2 = 1;
    __no_operation();
    __no_operation();
    __no_operation();
    __no_operation();
    PD_ODR_ODR2 = 0;
    WWDG_CR = 0xc1;
    WWDG_WR = 0x40;
}

This means that for the first 768uS the control register should not be changed. If the register is changed during this period a reset will be triggered. To demonstrate this we will change the value in the control register immediately after the microcontroller has been initialised:

int main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    InitialisePorts();
    InitialiseWWDG();
    __enable_interrupt();
    //
    //  Main program loop.
    //
    while (1)
    {
        WWDG_CR = 0xc1;             //  Trigger a reset.
        __wait_for_interrupt();
    }
}

Deploying this application to the microcontroller results in the following trace on the oscilloscope:

Watchdog immediate reset

As you can see, the system is reset almost immediately (there is virtually no time between the pulse on PD2 and the reset line being pulled low).

Example 3 – Reset Within the Watchdog Window

Starting with the common code we initialise the Window Watchdog with the following method:

//--------------------------------------------------------------------------------
//
//  Initialise the Windows Watchdog.
//
void InitialiseWWDG()
{
    WWDG_CR = 0x5b;         //  Approx 70ms total window.
    WWDG_WR = 0x4c;         //  Approx 11.52ms window where cannot reset
    WWDG_CR_WDGA = 1;       //  Enable the watchdog.
}

This code defines a period of 11.52ms where we cannot reset the window watchdog counter followed by a period of 9.216ms during which the watchdog counter must be reset in order to prevent the microcontroller from being reset.

A simple main application loop would look something like this:

//--------------------------------------------------------------------------------
//
//  Main program loop.
//
int main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    InitialisePorts();
    PD_ODR_ODR4 = 1;
    __no_operation();
    PD_ODR_ODR4 = 0;
    InitialiseWWDG();
    __enable_interrupt();
    //
    //  Main program loop.
    //
    while (1)
    {
        unsigned char counter = (unsigned char) WWDG_CR;
        if ((counter & 0x7f) < WWDG_WR)
        {
            WWDG_CR = 0xdb;     //  Reset the Window Watchdog counter.
            PD_ODR_ODR2 = !PD_ODR_ODR2;
        }
        //
        //  Do something here.
        //
    }
}

The initial pulse on PD4 indicates that the application has started. We can use this to detect the reset of the microcontroller. In this trivial application the main program loop simply checks to see if the current value of the counter is less than the value in WWDG_WR. If the counter is less than WWDG_WR then the system write a new value into the counter. The value written is 0x5b anded with 0x80, this brings the reset value into the value range (0xc0 – 0xff).

This application also pulses PD2 to indicate that the watchdog counter has been reset.

Deploying this application and hooking up the Saleae logic analyser gives the following trace:

Window watchdog initial trace

As you can see, there is an initial pulse showing that the board is reset (top trace) and then a series of pulses showing that the counter is being reset (lower trace). Each up/down transition represents a watchdog counter reset.

This is a relatively trivial example so let’s spice this up and add in a timer.

To the code above add the following code:

//--------------------------------------------------------------------------------
//
//  Setup Timer 2 to generate a 12.5ms interrupt.
//
void SetupTimer2()
{
    TIM2_PSCR = 0x02;       //  Prescaler = 4.
    TIM2_ARRH = 0xc3;       //  High byte of 50,000.
    TIM2_ARRL = 0x50;       //  Low byte of 50,000.
    TIM2_IER_UIE = 1;       //  Enable the update interrupts.
    TIM2_CR1_CEN = 1;       //  Finally enable the timer.
}

This method will set up timer 2 to generate an interrupt every 12.5ms.

Adding the following will catch the interrupt:

//--------------------------------------------------------------------------------
//
//  Timer 2 Overflow handler.
//
#pragma vector = TIM2_OVR_UIF_vector
__interrupt void TIM2_UPD_OVF_IRQHandler(void)
{
    if (_firstTime)
    {
        InitialiseWWDG();
        _firstTime = 0;
    }
    else
    {
        unsigned char counter = (unsigned char) WWDG_CR;
        unsigned char window = WWDG_WR;
        BitBangByte(counter & 0x7f);
        BitBangByte(window);
        WWDG_CR = 0xdb;     //  Reset the Window Watchdog counter.
        counter = (unsigned char) WWDG_CR;
        BitBangByte(counter);
    }
    PD_ODR_ODR2 = !PD_ODR_ODR2;
    TIM2_SR1_UIF = 0;       //  Reset the interrupt otherwise it will fire again straight away.
}

The interrupt will, on first invocation, initialise the window watchdog. Subsequent invocations will output the values of the registers and reset the window watchdog.

We need some code to bit bang the register values:

#define SR_CLOCK            PD_ODR_ODR5
#define SR_DATA             PD_ODR_ODR3

//
//  BitBang the data through the GPIO ports.
//
void BitBangByte(unsigned char b)
{
    //
    //  Initialise the clock and data lines into known states.
    //
    SR_DATA = 0;                    //  Set the data line low.
    SR_CLOCK = 0;                   //  Set the clock low.
    //
    //  Output the data.
    //
    for (int index = 7; index >= 0; index--)
    {
        SR_DATA = ((b >> index) & 0x01);
        SR_CLOCK = 1;               //  Send a clock pulse.
        __no_operation();
        SR_CLOCK = 0;
    }
    //
    //  Set the clock and data lines into a known state.
    //
    SR_CLOCK = 0;                   //  Set the clock low.
    SR_DATA = 0;
}

The main program loop needs to be modified to set up the timer and registers etc. So replace the main program loop with the following:

//--------------------------------------------------------------------------------
//
//  Main program loop.
//
int main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    InitialisePorts();
    PD_ODR_ODR4 = 1;
    __no_operation();
    PD_ODR_ODR4 = 0;
    SetupTimer2();
    InitialiseWWDG();
    __enable_interrupt();
    //
    //  Main program loop.
    //
    while (1)
    {
        __wait_for_interrupt();
    }
}

Deploying and running this application gives the following output on the Saleae logic analyser:

Window watchdog full trace

Zooming in produces the following

Window watchdog zoomed in

Starting with the top trace and descending we can see the following values:

• Reset pulse
• Timer 2 interrupt triggers (up/down transitions)
• Data (register values)
• Clock signal

The decoded register values can be seen above the data trace. The first value is the current value of the counter. The second value is the value in the window watchdog register and the final value is the new value in the counter register.

Conclusion

The Window Watchdog provides a mechanism for the developer to detect software faults similar to the Independent Watchdog but further constrains the developer by defining a window where a counter reset by the application is not allowed.

The beep function uses the 128 KHz LSI to generate a beep signal on the beep pin of the STM8S. This post demonstrates how to use this function.

Hardware

The signal is output on the beep pin. On the STM8S discovery board this is an alternative function on PD4 (port D, pin 4).

Setting the Alternative Function

The alternative functions are assigned to a pin by reprogramming the option bytes. This can be performed using the ST Visual Programmer. Start this application and read the option bytes from the STM8S:

Reading the options bytes using ST Visual Developer

1. Select the STM8S model. On the STM8S Discovery board this is STM8S105C6
2. Select the OPTION BYTE tab
3. Read the option bytes from the microcontroller
4. Check the value of the AFR7 bit

AFR7 needs to be set to Port D4 Alternative Function = BEEP on the STM8S Discovery board. This may be different on your STM8S so check the documentation for your microcontroller. To set the alternative function to beep:

Writing new options bytes using ST Visual Developer

1. Select Port D4 Alternative Function = BEEP from the list of values in the alternative functions
2. Write the option bytes back to the microcontroller

The beep function should now be assigned to PD4.

Registers

The beep function is controlled by three registers:

• BEEP_CSR_BEEPSEL
• BEEP_CSR_BEEPDIV
• BEEP_CSR_BEEPEN

Beep Selection – BEEP_CSR_BEEPSEL

These bits set the multiplier for the Beep Divider.

Beep Divider – BEEP_CSR_BEEPDIV

Value for the prescalar divider.

BEEP_DIV = 2 + BEEP_CSR_BEEPDIV

The documentation states that this should not be left set to the reset value (0x1f) and the value should be in the range 0 – 0x1e.

Beep Enable – BEEP_CSR_BEEPEN

Enable the beep function by setting this to 1, disable by setting this to 0.

Software

We start by providing a method for resetting the system clock to use the HSI:

//
//  This program demonstrates how to use the beep function on the STM8S 
//  microcontroller.
//
//  This software is provided under the CC BY-SA 3.0 licence.  A
//  copy of this licence can be found at:
//
//  http://creativecommons.org/licenses/by-sa/3.0/legalcode
//
#include <iostm8S105c6.h>
#include <intrinsics.h>

//--------------------------------------------------------------------------------
//
//  Setup the system clock to run at 16MHz using the internal oscillator.
//
void InitialiseSystemClock()
{
    CLK_ICKR = 0;                       //  Reset the Internal Clock Register.
    CLK_ICKR_HSIEN = 1;                 //  Enable the HSI.
    CLK_ECKR = 0;                       //  Disable the external clock.
    while (CLK_ICKR_HSIRDY == 0);       //  Wait for the HSI to be ready for use.
    CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.
    CLK_PCKENR1 = 0xff;                 //  Enable all peripheral clocks.
    CLK_PCKENR2 = 0xff;                 //  Ditto.
    CLK_CCOR = 0;                       //  Turn off CCO.
    CLK_HSITRIMR = 0;                   //  Turn off any HSIU trimming.
    CLK_SWIMCCR = 0;                    //  Set SWIM to run at clock / 2.
    CLK_SWR = 0xe1;                     //  Use HSI as the clock source.
    CLK_SWCR = 0;                       //  Reset the clock switch control register.
    CLK_SWCR_SWEN = 1;                  //  Enable switching.
    while (CLK_SWCR_SWBSY != 0);        //  Pause while the clock switch is busy.
}

The next task is to configure the beep function:

//--------------------------------------------------------------------------------
//
//  Initialise the Beep function.
//
void InitialiseBeep()
{
    BEEP_CSR_BEEPEN = 0;    //  Turn off the beep.
    BEEP_CSR_BEEPSEL = 0;   //  Set beep to fls / (8 * BEEPDIV) KHz.
    BEEP_CSR_BEEPDIV = 0;   //  Set beep divider to 2.
    BEEP_CSR_BEEPEN = 1;    //  Re-enable the beep.
}

The final piece of code is the main program loop. This sets up the clock and the beep function:

//--------------------------------------------------------------------------------
//
//  Main program loop.
//
void main()
{
    //
    //  Initialise the system.
    //
    __disable_interrupt();
    InitialiseSystemClock();
    InitialiseBeep();
    __enable_interrupt();
    //
    //  Main program loop.
    //
    while (1)
    {
        __halt();
    }
}

Compiling this code and deploying to the STM8S results in the following trace on the oscilloscope:

Beep Oscilloscope Output

As you can see, the output is not exactly 8KHz.

Conclusion

The documentation states that the beep function can generate 1KHz, 2KHz and 4KHz. By changing the values of the prescalar and the selection register it appears that you can also go as low as 500Hz and as high as 32KHz.