Embedded Developer

A couple of days ago I was working on a project and was having some problems debugging. It was nothing to do with the debugging environment more to do with the high speed interaction between two devices. So I needed some debug output – enter UART1. I decided that I would send some debug information to UART1 and capture this in PuTTY. At least I would be able to see the sequence of events and hopefully work out where my code is going wrong.

This is where my problems started…

UART Examples

So I started looking through the Standard Peripheral Library example programs. Sure enough there is a program there in the directory UART1_Printf Sounds ideal, I should be able to throw some text data at this and have it appear in PuTTY. Great.

Quick check of the readme.txt in this directory and it looks like this program is compatible:

Hardware and Software environment

This example runs on STM8S High density and Low density devices and on STM8A High density devices.

So I am running on the STM8S103F3 which is one of the low density STM8 devices so I should be fine with this code. A little further down the file it gives the specification for the serial comms as:

Hyperterminal configuration:

• Word Length = 8 Bits
• One Stop Bit
• No parity
• BaudRate = 115200 baud
• flow control: None

Great – we are in business!

So what happened?

Creating a project was simple and I had the code compiling within a few minutes. Next job is to hook up PuTTY and send some data through down the serial connection and see it appear on the PC. This is where life got a little messy. Nothing appeared. Double checked all of the settings in both the program and Putty – they both match. Ahhh – should Tx and Rx be crossed? No that did not work.

Hmmmmmmmm….. This could be a long day.

Next step, break out the trusty logic analyser from Saleae. Connected this up and managed to show that data was in fact being output from the application but at 53 baud. This is a long way from the 115200 baud promised.

Hmmmm, so lets change the baud rate requested – 9600 baud is normally safe. No change, still 53 baud.

Starts clutching at straw, lets just make sure the system clock is running using the HSI 16 MHz signal. Still no joy, 53 baud is all I seem to be getting.

The Problem

I finally broke out the data sheet for the chip and started single stepping through the STD Peripheral library code. The example program contains the following line of code:

UART1_Init((uint32_t)115200, UART1_WORDLENGTH_8D, UART1_STOPBITS_1, UART1_PARITY_NO,
UART1_SYNCMODE_CLOCK_DISABLE, UART1_MODE_TXRX_ENABLE);

This should set up the port to operate at the rates as per the read me file. If you have a look at this method, about half way down you will see the following code:

/* Set the UART1 BaudRates in BRR1 and BRR2 registers according to UART1_BaudRate value */
BaudRate_Mantissa    = ((uint32_t) CLK_GetClockFreq() / (BaudRate << 4));
BaudRate_Mantissa100 = (((uint32_t) CLK_GetClockFreq() * 100) / (BaudRate << 4));
/* Set the fraction of UART1DIV  */
UART1->BRR2 |= (uint8_t)((uint8_t)(((BaudRate_Mantissa100 - (BaudRate_Mantissa * 100)) << 4) / 100) & (uint8_t)0x0F);
/* Set the MSB mantissa of UART1DIV  */
UART1->BRR2 |= (uint8_t)((BaudRate_Mantissa >> 4) & (uint8_t) 0xF0);
/* Set the LSB mantissa of UART1DIV  */
UART1->BRR1 |= (uint8_t) BaudRate_Mantissa;

So I am single stepping through the code and notice something odd, the Baud Rate Registers (BRR1 and BRR2) are not being set correctly. By my calculations these should be 0x68 and 0x03 respectively (for 9600 baud).

A quick modification to the source file to put these explicit changes in and….

It Works !

I now have PuTTY talking to my STM8 at 9600 baud and so my debugging life should be a little easier. On the downside, I have lost a little faith in the Standard Peripheral Library. I think that in future I will be using this more for guidance rather than as a method of programming this family of processors. Coupled with the data sheet for the chip is is going to be an invaluable tool. Going forward I will be building up my own set of methods working with the registers on the processor. This may be slower but it will bring me closer to the chip and I may occasionally come across the odd gem in the data sheet which I may not otherwise have become aware of.

Now, back to the original problem.

Tags: STM8