Port of https://github.com/cpldcpu/CandleLEDhack/ for STM8/SDCC (Requires STM8 SPL)

CandleflickerLED.c

/*
 * CandeflickerLED.c
 *
 * Created: 01.20.2021 
 * 
 * Emulates a Candleflicker-LED on an STM8 microcontroller.
 * Please use SDCC to compile and update the defines below according to your controller.
 * (Require the ST SPL library, patched for SDCC): https://github.com/gicking/STM8-SPL_SDCC_patch
 *
 * Original code and research here: 
 * https://github.com/cpldcpu/CandleLEDhack/
 *
 * Ported to STM8 by Brad Roy
 */

#define   F_CPU 16000000

#include <stm8s.h>

void delay_us(uint16_t microseconds) {
    TIM4->PSCR = TIM4_PRESCALER_1; // Set prescaler

    // Set count to approximately 1uS (clock/microseconds in 1 second)
    // The -1 adjusts for other runtime costs of this function
    TIM4->ARR = ((16000000L)/1000000) - 1;

    TIM4->CR1 = TIM4_CR1_CEN; // Enable counter

    for (; microseconds > 1; --microseconds) {
        while ((TIM4->SR1 & TIM4_SR1_UIF) == 0);

        // Clear overflow flag
        TIM4->SR1 &= ~TIM4_SR1_UIF;
    }
}

/*
        32 Bit maximum length LFSR
        see http://www.ece.cmu.edu/~koopman/lfsr/index.html
	
	    Using inverted values so the LFSR also works with zero initialisiation.
*/
uint8_t Rand(void) {
	static uint32_t Z;
	
	  if (Z & 1)  { Z = (Z >> 1); }
	  else        { Z = (Z >> 1) ^ 0x7FFFF159; }
	
	return (uint8_t)Z;
}

// Led connected to PD4
#define LEDPORT GPIOD
#define LEDPIN  GPIO_PIN_4

int main(void)
{
	CLK_HSIPrescalerConfig(CLK_PRESCALER_HSIDIV1); //set CPU to 16MHz
		
	uint8_t PWM_CTR=0;		// 4 bit-Counter
	uint8_t FRAME_CTR=0;		// 5 bit-Counter
	
	uint8_t PWM_VAL=0;		// 4 bit-Register
	uint8_t NEXTBRIGHT=0;		// 4 bit-Register
	uint8_t RAND=0;			// 5 bit Signal
	uint8_t randflag=0;		// 1 bit Signal
	
	GPIO_Init(LEDPORT,LEDPIN,GPIO_MODE_OUT_PP_LOW_SLOW);
	
    while(1)        
      {				
		delay_us(1e6/440/16);   // Main clock=440*16 Hz
		
		// PWM		
		PWM_CTR++;
		PWM_CTR&=0xf;		// only 4 bit
		
		if (PWM_CTR<=PWM_VAL) {GPIO_WriteHigh(LEDPORT,LEDPIN);} else {GPIO_WriteReverse(LEDPORT,LEDPIN);}  

		// FRAME
		if (PWM_CTR==0) 
		{
			FRAME_CTR++;
			FRAME_CTR&=0x1f;
			
			if ((FRAME_CTR&0x07)==0)  // generate a new random number every 8 cycles. In reality this is most likely bit serial
			{
				RAND=Rand()&0x1f;  				
				if ((RAND&0x0c)!=0) randflag=1; else randflag=0;// only update if valid 				
			}
			
			// NEW FRAME						
			if (FRAME_CTR==0)
			{
				PWM_VAL=NEXTBRIGHT; // reload PWM
				randflag=1;		    // force update at beginning of frame
			}					
			
			if (randflag)
			{
				NEXTBRIGHT=RAND>15?15:RAND;	 	
			}
		}
    }
}