使用STM32L TIM PWM控制伺服电机

Question

So I'm trying to control a servo by PWM using STM32L1. below is the full code, no library needed. When I upload and run this code, the servo ticks several times instead of actually moving smoothly to the required positions 600(0*), 1100(50*), 1600(100*), and 2100(150*). I believe this is related to the prescale value calculation, plus I'm not sure whether mentioning the ARR in micro-seconds is correct, if not, how do I configure it to read microseconds instead of milliseconds. Please refer to the notes in the code for further details.

#include <stdio.h>
#include "stm32l1xx.h"             
 // Keil::Device:Startup

 // initialization of GPIOB
void TIM4_Init(){
RCC->AHBRSTR |= RCC_AHBRSTR_GPIOARST;       /* Reset GPIOB clock         */
RCC->AHBENR |= RCC_AHBENR_GPIOBEN;           /* Enable GPIOB clock         */
GPIOB->MODER   &=   ~(0x03 << (2*6));       /* Clear bit 11 & 12 Alternate mode*/
GPIOB->MODER   |=   0x02 << (2*6);          /* set as Alternate mode*/
GPIOB->OSPEEDR &=   ~(0x03<< (2*6));        /* 40 MHz  speed        */
GPIOB->OSPEEDR |=   0x03<< (2*6);           /* 40 MHz  speed        */
GPIOB->PUPDR &=         ~(1<<6);            /* NO PULL-UP PULL-DOWN        */
GPIOB->OTYPER &=        ~(1<<6);            /* PUSH-PULL        */
GPIOB->AFR[0] |=        0x2 << (4*6);       /* Pin6 set as alternate function 2 (TIM4) */

RCC->APB1ENR |= RCC_APB1ENR_TIM4EN;
TIM4->PSC = 16; //prescale value, AHB/APB1 Fmax=32MHz / 2
TIM4->ARR = 20000-1; //motor Freq = 50Hz, Period(ARR)= 1/50 = 20000us


// initialization of TIM & PWM

TIM4->CCMR1 |= TIM_CCMR1_OC1M;  // 111: PWM mode 2 - In upcounting, channel 1 is inactive 
                               //as long as TIMx_CNT<TIMx_CCR1 else active. 
                              // In downcounting, channel 1 is active as long as 
                             //TIMx_CNT>TIMx_CCR1 else inactive.

TIM4->CCMR1 |= TIM_CCMR1_OC1PE;
TIM4->CR1 |= TIM_CR1_ARPE;
TIM4->CCER |= TIM_CCER_CC1E;
TIM4->EGR |= TIM_EGR_UG;
TIM4->SR &= ~TIM_SR_UIF;
TIM4->DIER |= TIM_DIER_UIE;
TIM4->CR1 |= TIM_CR1_CEN;
}

// set servo to 4 positions in sequence

int main(void){

int i;

int position=600; // initial motor position

TIM4_Init();

while (1){  

if ((position >=600)|| (position <=2100))
position = position+500;      // motor positions will be 600(0*), 1100(50*), 1600(100*)
                             //, 2100(150*)
TIM4->CCR1 = position; 

for(i=0;i<1000;i++); // short delay


   }


 }  

Answer 1

That depends on your clock settings. You need to configure your clock first as many L1 have the default clock after the reset 2.097MHz only and the period with your settings is 0.152598951 sek, and the pulse width received by the servo is 0.07 to 0.15sek which is ~75times to long (I presume that PCS should be 15 to archive 16*20000 clocks period). With your setting the timer clock should be more than 100MHz which is impossible for L1

Timers count ticks and how long the tick takes depends on the clock speed, and timer clock dividers.

To archive 20ms you need to leave PSC with zero value and ARR = 41939

Of course do 1ms will be 2097 and 2ms - 4194

You need to calculate the correct values

The easiest way to configure PLL is using the WYSIWYG CubeMX clock editor

Answer 2

It really depends on your clock settings, as @PeterJ mentioned. I will just put some more precision here.

Your ARR seems fine, as it's clear to read that your period is 20ms.

But your PSC then should be set depending on the ARR in order to reach 1kHz. For that you have to know the clock frequency.

A side note, the PSC value should be 1 unit less than desired (like if you want 16, you should write 15 to the register), according to the stm documentation.

The counter clock frequency CK_CNT is equal to fCK_PSC / (PSC[15:0] + 1)