openpilot_comma/panda/board/drivers/fan.h

void fan_set_power(uint8_t percentage){
  pwm_set(TIM3, 3, percentage);
}

uint16_t fan_tach_counter = 0U;
uint16_t fan_rpm = 0U;

// Can be way more acurate than this, but this is probably good enough for our purposes.

// Call this every second
void fan_tick(void){
    // 4 interrupts per rotation
    fan_rpm = fan_tach_counter * 15U;
    fan_tach_counter = 0U;
}

// TACH interrupt handler
void EXTI2_IRQ_Handler(void) {
    volatile unsigned int pr = EXTI->PR & (1U << 2);
    if ((pr & (1U << 2)) != 0U) {
        fan_tach_counter++;
    }
    EXTI->PR = (1U << 2);
}

void fan_init(void){
    // 5000RPM * 4 tach edges / 60 seconds
    REGISTER_INTERRUPT(EXTI2_IRQn, EXTI2_IRQ_Handler, 700U, FAULT_INTERRUPT_RATE_TACH)

    // Init PWM speed control
    pwm_init(TIM3, 3);

    // Init TACH interrupt
    register_set(&(SYSCFG->EXTICR[0]), SYSCFG_EXTICR1_EXTI2_PD, 0xF00U);
    register_set_bits(&(EXTI->IMR), (1U << 2));
    register_set_bits(&(EXTI->RTSR), (1U << 2));
    register_set_bits(&(EXTI->FTSR), (1U << 2));
    NVIC_EnableIRQ(EXTI2_IRQn);
}