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Getting started with STM8 development using free software: Serial output on the STM8L-DISCOVERY

This short tutorial presents a simple "Hello World" program for the STMicroelectronics STM8L-DISCOVERY board, with an extra WaveShare RS232 board. The author used a Debian GNU/Linux system, but the tutorial should work for other Linux distributions, *BSD or other Unices.

The tools we use are

Hardware setup

STM8L-DISCOVERY with Demo running

The STM8L-DISCOVERY is connected to the host computer via a USB cable for power and to write the demo onto the board using the integrated stlink. We connect the RS232 board to power pins and for data to the I/O pins (TxIN to PC3, RxOUT to PC2); the serial cable is then attached to the RS232 board. On the other end the serial cable is attached to an RS232 port on a computer running a terminal program configured for 9600 baud, no parity, 8 bits, 1 stop bit and no flow control. We used a USB-to-serial converter and gtkterm.


Depending on your operating system there might be an easy way to install SDCC 3.5.0 or newer using a package system or similar (e.g. apt-get install sdcc on Debian). While SDCC 3.4.0 should be sufficient for this tutorial, you might want to try a newer version in case you encounter any bugs. In particular, SDCC 3.4.0 has an issue with the library search path; this can be worked around by explicitly specifying the path to the standard library when linking.

SDCC binaries or a source tarball can be downloaded from its website.

Get stm8flash

The stm8flash source can be found at its GitHub location, where there is also a download link for a zip archive of the sources. To compile it, a C compiler, such as gcc, pkg-config and libusb need to be installed. Unzip the archive (e.g. using unzip change into the directory stm8flash-master and type make. In case there are any errors, such as header files not found, check that pkg-config and development files for libusb are installed.

The Demo

We present a simple Demo that repeatedly outputs "Hello World!" on UART2 (the only UART the STM8S105 has). This demonstrates setting up and using the UART for serial I/O. Here is the C code:

// Source code under CC0 1.0
#include <stdint.h>
#include <stdio.h>

#define PC_DDR	(*(volatile uint8_t *)0x500c)
#define PC_CR1	(*(volatile uint8_t *)0x500d)

#define CLK_DIVR	(*(volatile uint8_t *)0x50c0)
#define CLK_PCKENR1	(*(volatile uint8_t *)0x50c3)

#define USART1_SR	(*(volatile uint8_t *)0x5230)
#define USART1_DR	(*(volatile uint8_t *)0x5231)
#define USART1_BRR1	(*(volatile uint8_t *)0x5232)
#define USART1_BRR2	(*(volatile uint8_t *)0x5233)
#define USART1_CR2	(*(volatile uint8_t *)0x5235)
#define USART1_CR3	(*(volatile uint8_t *)0x5236)

#define USART_CR2_TEN (1 << 3)
#define USART_CR3_STOP2 (1 << 5)
#define USART_CR3_STOP1 (1 << 4)
#define USART_SR_TXE (1 << 7)

void putchar(char c)
	while(!(USART1_SR & USART_SR_TXE));

	USART1_DR = c;

void main(void)
	unsigned long i = 0;

	CLK_DIVR = 0x00; // Set the frequency to 16 MHz
	CLK_PCKENR1 = 0xFF; // Enable peripherals

	PC_DDR = 0x08; // Put TX line on
	PC_CR1 = 0x08;

	USART1_CR2 = USART_CR2_TEN; // Allow TX and RX
	USART1_CR3 &= ~(USART_CR3_STOP1 | USART_CR3_STOP2); // 1 stop bit
	USART1_BRR2 = 0x03; USART1_BRR1 = 0x68; // 9600 baud

		printf("Hello World!\n");
		for(i = 0; i < 147456; i++); // Sleep

sdcc is a freestanding, not a hosted implemenatation of C, and allows main to return void. Note that currently putchar() returns void. This is not standard compliant and might change to int in future sdcc versions. The printf() from the standard library uses putchar() for output. Since putchar() is device-specific we need to supply it. In this case we want it to output data using UART1.

The demo can be compiled simply by invocing sdcc using sdcc -mstm8 --std-c99 serial.c assuming the C code is in serial.c. The option -mstm8 selects the target port (stm8). An .ihx file with a name corresponding to the source file will be generated.

Put the demo onto the board

Assuming stm8flash and serial.ihx are in the same directory, the board is attached through the integrated stlink device, ./stm8flash -c stlink -p stm8l152c6 -w serial.ihx will write the demo onto the board. You can see the "Hello world" by attaching a serial cable to the DB9 connector on the RS232 board, and using a terminal program configured for 9600 baud, no parity, 8 bits, 1 stop bit and no flow control.

More about stm8flash

stm8flash was written by Valentin Dudouyt. It works both with stlink (including the one integrated on the discovery boards) and stlinkv2 devices. The programmer can be selected using -c stlink or -c stlinkv2. The target device is selected using the -p option (to get a list of target devices, use the -p option with an option argument that is not an stm8 device, e.g. -p help. stm8flash will treat filenames ending in .ihx a Intel hex, and other filenames as binaries.

More about SDCC

SDCC was initially written by Sandeep Dutta for the MCS-51, and has a relatively conservative architecture (see Sandeep Dutta, "Anatomy of a Compiler", 2000). It has been extended by various contributors and more recently, incorporated some cutting-edge technologies, in particular in register allocation (see Philipp Klaus Krause, "Optimal Register Allocation in Polynomial Time", 2013). The stm8 backend was mostly written by Philipp Klaus Krause for his research into bytewise register allocation and spilling (see Philipp Klaus Krause, "Bytewise Register Allocation", 2015).

SDCC is a C compiler that aims to be compliant with the C standards.

Important compiler options for STM8 developers include: