How to control a servo motor using STM32F103C8T6 timers?

Question

I am trying to control the position of a servo motor using an STM32F103C8T6 MCU (blue pill). The problem I am facing is that the servo does not move to different positions when given different values (in a infinite loop.)

I am using bare-metal programming so that I can understand the basic at hardware level.

Here is my code:

// ------------ THIS Code WORKS ------------------------------------------
#include "stm32f10x.h"                  // Device header

void delayMs(void);
void systick_init(void);
void delayMillis(int);
void writeMicroseconds(int);

int main(void){

    RCC->APB1ENR |= RCC_APB1ENR_TIM4EN; //enable clock for TIM4
    RCC->APB2ENR |= RCC_APB2ENR_IOPBEN | RCC_APB2ENR_AFIOEN; //enable clock for PORTB and enable AFIO

    //cofigure PB6 as pwm output CNF6 = 10 (alternate fnc output push pull) MODE6 = 11 (output 50MHz)
    GPIOB->CRL = 0x4B444444;//(1<<25)|(1<<24)|(1<<27)|(0<<26);
    
    //configure pwm as active high and enable the signal 
    TIM4->CCER |= 1;
    
    TIM4->CR1 |= (1<<7);//Auto reload preload enable ARPE
    
    //timer config for pwm mode
    //CH1 > OC1M = 110 & ocipe = 1
    TIM4->CCMR1 |= (1<<6)|(1<<5)|(1<<3);
    
    //Timer config for PSC and ARR
    // clk = 72000000 : need 50Hz => 50 = 72000000/x/20000; => x = 72
    TIM4->PSC = 72 - 1;
    TIM4->ARR = 20000;
    
    //enable all registers by UG bit 
    TIM4->EGR |= 1;
    TIM4->CR1 |= 1; //enable timer CEN = 1; 
    
    writeMicroseconds(1500); // SERVO gets set to 90° perfectly
      

}
void writeMicroseconds(int32_t pwm){    
    TIM4->CCR1 = pwm;
}   

//systick timer config
void systick_init(void){
    SysTick->CTRL = 0;
    SysTick->LOAD = 0x00ffffff;
    SysTick->VAL = 0;
    
    SysTick->CTRL = 5;
}

//single millisecond control config
void delayMs(void){
    SysTick->LOAD = 72000-1;
    SysTick->VAL = 0;
        while((SysTick->CTRL & 0x00010000) == 0);
}

//given millisecond delay fnc
void delayMillis(int t){
    for(;t>0;t--){
        delayMs();
    }
}

// ------------ THIS Code DOES NOT ------------------------------------------

#include "stm32f10x.h"                  // Device header

void delayMs(void);
void systick_init(void);
void delayMillis(int);
void writeMicroseconds(int);

int main(void){

    RCC->APB1ENR |= RCC_APB1ENR_TIM4EN; //enable clock for TIM4
    RCC->APB2ENR |= RCC_APB2ENR_IOPBEN | RCC_APB2ENR_AFIOEN; //enable clock for PORTB and enable AFIO

    //cofigure PB6 as pwm output CNF6 = 10 (alternate fnc output push pull) MODE6 = 11 (output 50MHz)
    GPIOB->CRL = 0x4B444444;//(1<<25)|(1<<24)|(1<<27)|(0<<26);
    
    //configure pwm as active high and enable the signal 
    TIM4->CCER |= 1;
    
    TIM4->CR1 |= (1<<7);//Auto reload preload enable ARPE
    
    //timer config for pwm mode
    //CH1 > OC1M = 110 & ocipe = 1
    TIM4->CCMR1 |= (1<<6)|(1<<5)|(1<<3);
    
    //Timer config for PSC and ARR
    // clk = 72000000 : need 50Hz => 50 = 72000000/x/20000; => x = 72
    TIM4->PSC = 72 - 1;
    TIM4->ARR = 20000;
    
    //enable all registers by UG bit 
    TIM4->EGR |= 1;
    TIM4->CR1 |= 1; //enable timer CEN = 1; 
    
    while(1){ // In this loop the servo gets fixed at one position and then doesn't move at all! even though I Change CCR1 value.
      writeMicroseconds(1500);
      delayMillis(500);
      writeMicroseconds(1050);
      delayMillis(500);
      writeMicroseconds(1500);
      delayMillis(500);
      writeMicroseconds(2000);
      delayMillis(500);
      writeMicroseconds(1500);
      delayMillis(500);
      writeMicroseconds(1050);
      delayMillis(500);
    }

}
void writeMicroseconds(int32_t pwm){    
    TIM4->CCR1 = pwm;
}   

//systick timer config
void systick_init(void){
    SysTick->CTRL = 0;
    SysTick->LOAD = 0x00ffffff;
    SysTick->VAL = 0;
    
    SysTick->CTRL = 5;
}

//single millisecond control config
void delayMs(void){
    SysTick->LOAD = 72000-1;
    SysTick->VAL = 0;
        while((SysTick->CTRL & 0x00010000) == 0);
}

//given millisecond delay fnc
void delayMillis(int t){
    for(;t>0;t--){
        delayMs();
    }
}

I did this using the reference manual available in Keil MDK also here - Chapter 15. In the manual under PWM mode, an approach is given which I followed.

This is what is happening:

The servo gets fixed at a certain PWM (probably the fist value given to CCR1 initially). Even though I change the value of CCR1 register in the while loop, the servo stays fixed.

I tried with individual PWM widths and it works fine. But I want to change the PWM width in a loop - whatever value I pass to writeMicroseconds() , the servo should get to that position; that's not happening in my code.

There is no particular reference to do this. I did it only on the basis of the manual and now I don't understand what I did wrong.

Answer 1

Systick is not initialized so delays do not work.