Continuing the STM8 Expedition

STM8S105 Discovery

True Data EEPROM

There are several cases where we need to save some very important data in a microcontroller and have them retrieved later. For instance, we would like keep preferences like contrast, colour, etc. of our TV set fixed once set and would like the TV to remember them whenever we turn it on. Usually a micro has three types of memory – RAM, EEPROM and flash memory. EEPROM and flash memory are non-volatile memories, meaning they retain the data stored in them even when powered off. RAM, on the other hand, is a volatile memory. All these memories have different roles. RAM is mainly intended for fast processing and temporary storage. EEPROMs are intended for storing a small set of important data that are often needed to be recalled. Finally, there is flash memory to hold the application code. Thus, it is logical to store settings and calibration data in either EEPROM or flash memory.

STM8s have true data EEPROMs and flash memories. Both share same registers and controls. Shown below is the memory map of STM8S003K3 microcontroller:

 Memory Map

You can find this info in your device’s datasheet. Check the address ranges. This is important because if you try to write or read wrong locations, your application code will crash as it is an illegal operation.


Hardware Connection

 Hardware Schematic



Code Example

#include "STM8S.h"
#include "lcd.h"

#define Button_Port              GPIOB
#define Button_Pin               GPIO_PIN_7

unsigned char bl_state;
unsigned char data_value;

void clock_setup(void);
void GPIO_setup(void);
void Flash_setup(void);
void lcd_print(unsigned char x_pos, unsigned char y_pos, unsigned char value);

void main()
       unsigned char value = 0x00;



       LCD_goto(0, 0);
       LCD_putstr("STM8 EEPROM Test");

       value = FLASH_ReadByte(0x4000);

       LCD_goto(0, 1);
       LCD_putstr("WR: ---");
       LCD_goto(9, 1);
       lcd_print(13, 1, value);

              if(GPIO_ReadInputPin(Button_Port, Button_Pin) == FALSE)
                     while(GPIO_ReadInputPin(Button_Port, Button_Pin) == FALSE);
                     FLASH_ProgramByte(0x4000, value);

                     lcd_print(13, 1, value);

              lcd_print(4, 1, value);


void clock_setup(void)

       while(CLK_GetFlagStatus(CLK_FLAG_HSIRDY) == FALSE);



       CLK_PeripheralClockConfig(CLK_PERIPHERAL_I2C, ENABLE);
       CLK_PeripheralClockConfig(CLK_PERIPHERAL_SPI, DISABLE);
       CLK_PeripheralClockConfig(CLK_PERIPHERAL_ADC, DISABLE);
       CLK_PeripheralClockConfig(CLK_PERIPHERAL_AWU, DISABLE);
       CLK_PeripheralClockConfig(CLK_PERIPHERAL_UART1, DISABLE);
       CLK_PeripheralClockConfig(CLK_PERIPHERAL_TIMER1, DISABLE);
       CLK_PeripheralClockConfig(CLK_PERIPHERAL_TIMER2, DISABLE);
       CLK_PeripheralClockConfig(CLK_PERIPHERAL_TIMER4, DISABLE);

void GPIO_setup(void)
       GPIO_Init(Button_Port, Button_Pin, GPIO_MODE_IN_FL_NO_IT);

void Flash_setup(void)

void lcd_print(unsigned char x_pos, unsigned char y_pos, unsigned char value)
       char chr = 0x00;

       chr = ((value / 100) + 0x30);
       LCD_goto(x_pos, y_pos);

       chr = (((value / 10) % 10) + 0x30);
       LCD_goto((x_pos + 1), y_pos);

       chr = ((value % 10) + 0x30);
       LCD_goto((x_pos + 2), y_pos);



In this demo, two-wire I2C LCD is used for displaying data. The code works by reading the last data stored in the EEPROM and incrementing a variable named value. Only one location (0x4000) is read and updated when the Discovery board’s button is pressed.

Using the internal EEPROM is very easy. We need to deinitialize it first:

void Flash_setup(void)


Reading a location is as follows:

value = FLASH_ReadByte(0x4000);

We just have to set the location we wish to read.

To erase and to write, we need to unlock the memory type we wish to write using Memory Access Security System (MASS) key, write/erase the desired location and the lock it back. This lock feature ensures safety from data corruption and accidental writes.

FLASH_ProgramByte(0x4000, value);




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    I have taken a look at ADC1_Init definition and I found it uses ADC1_ConversionConfig to set channels and conversion mode. So it seems using ADC1_Init once with all needed channels is enough. Am I right?
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  • why are you sending the received data back through TX pin?


    I am wondering how to get a Max31855 to talk to a STM8s via SPI.


    • What’s to wonder about it? It is a simple SPI communication and SPI for STM8 is no different from the SPI of other MCUs…. The following lines are taken from the device’s datasheet and the write up there states how to communicate with it:

      “Drive CS low to output the first bit on the SO pin. A complete serial-interface read of the cold-junction compensated thermocouple temperature requires 14 clock cycles. Thirty-two clock cycles are required to read both the thermocouple and reference junction temperatures (Table 2 and Table 3.) The first bit, D31, is the thermocouple temperature sign bit, and is presented to the SO pin within tDV of the falling edge of CS. Bits D[30:18] contain the converted temperature in the order of MSB to LSB, and are presented to the SO pin within tD0 of the falling edge of SCK. Bit D16 is normally low and goes high when the thermocouple input is open or shorted to GND or VCC. The reference junction temperature data begins with D15. CS can be taken high at any point while clocking out con-version data. If T+ and T- are unconnected, the thermocouple temperature sign bit (D31) is 0, and the remainder of the thermocouple temperature value (D[30:18]) is 1.”

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