Timers and PWM (Bare-Metal)

Purpose

Configure MCU timers for PWM output, input capture, and periodic scheduling: prescaler/ARR setup, compare channels, and SysTick as a simple tick source.

When to Use

Motor/LED dimming via PWM
Measuring pulse width (input capture)
Bare-metal millis() without FreeRTOS
Hardware periodic sampling trigger

Workflow

1. PWM on TIM2 CH1 (STM32)

f_pwm = f_timer_clk / ((PSC+1) * (ARR+1))
duty = (CCR / (ARR+1)) * 100%

RCC->APB1ENR |= RCC_APB1ENR_TIM2EN;

TIM2->PSC  = 83;      /* 84MHz / 84 = 1 MHz tick */
TIM2->ARR  = 999;     /* 1 kHz PWM */
TIM2->CCR1 = 500;     /* 50% duty */
TIM2->CCMR1 |= (6U << TIM_CCMR1_OC1M_Pos);  /* PWM mode 1 */
TIM2->CCER  |= TIM_CCER_CC1E;
TIM2->CR1   |= TIM_CR1_CEN;

GPIO: PA0 AF1 TIM2_CH1.

2. SysTick tick

volatile uint32_t tick_ms;

void SysTick_Handler(void) {
    tick_ms++;
}

void systick_init(uint32_t hz) {
    SysTick_Config(SystemCoreClock / hz);
}

3. Input capture

Configure channel in CCMR as input, set CCER polarity, read CCR on interrupt when edge captured.

4. Agent usage

/timers-pwm-baremetal 1 kHz 25% duty PWM on TIM2 channel 1

Common Problems

Symptom	Cause	Fix
Wrong frequency	APB timer clock doubling	Check RM timer clock x2 rule
No PWM output	CCER/CCMR not enabled	Enable channel output
Jittery tick	SysTick overridden	One owner for SysTick

Related Skills

skills/baremetal/gpio-baremetal — timer pin AF
skills/embedded/freertos — replace SysTick with RTOS tick
skills/baremetal/adc-dac-baremetal — timer TRGO for ADC trigger