Low power test for Feather M0

main.c

// MIT license, Joey Castillo 2022
// works with Alex Taradov's bare metal SAM D21 project:
// https://github.com/ataradov/mcu-starter-projects

//-----------------------------------------------------------------------------
#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>
#include <string.h>
#include "samd21.h"
#include "hal_gpio.h"

//-----------------------------------------------------------------------------
#define BLINK_PERIOD_SECONDS 15

HAL_GPIO_PIN(LED, A, 17)

//-----------------------------------------------------------------------------
static void sys_init(void) {
    // Switch to 8MHz clock (disable prescaler)
    SYSCTRL->OSC8M.bit.PRESC = 0;
    // make sure main oscillator stays off in standby
    SYSCTRL->OSC8M.bit.RUNSTDBY = 0;

    // set up external 32k crystal oscillator
    SYSCTRL->XOSC32K.reg = SYSCTRL_XOSC32K_STARTUP(0x7) | SYSCTRL_XOSC32K_EN32K |
                           SYSCTRL_XOSC32K_XTALEN | SYSCTRL_XOSC32K_RUNSTDBY;
    SYSCTRL->XOSC32K.bit.ENABLE = 1;
    while(!SYSCTRL->PCLKSR.bit.XOSC32KRDY);

    // connect external crystal to GCLK1
    GCLK->GENDIV.reg = GCLK_GENDIV_ID(1) | GCLK_GENDIV_DIV(1);
    GCLK->GENCTRL.reg = GCLK_GENCTRL_ID(1) | GCLK_GENCTRL_SRC_XOSC32K |
                        GCLK_GENCTRL_IDC | GCLK_GENCTRL_GENEN;
    while(GCLK->STATUS.bit.SYNCBUSY);
}

//-----------------------------------------------------------------------------
static void timer_init(void) {
    PM->APBCMASK.reg |= PM_APBCMASK_TC3;

    // clock TC3 with GCLK1 (the 32k crystal)
    GCLK->CLKCTRL.reg = GCLK_CLKCTRL_ID(TC3_GCLK_ID) | GCLK_CLKCTRL_CLKEN |
                        GCLK_CLKCTRL_GEN(1);

    // Run in standby, divide 32768 Hz clock by 1024 (32 ticks per second)
    TC3->COUNT16.CTRLA.reg = TC_CTRLA_MODE_COUNT16 | TC_CTRLA_WAVEGEN_MFRQ |
                             TC_CTRLA_PRESCSYNC_RESYNC | TC_CTRLA_RUNSTDBY |
                             TC_CTRLA_PRESCALER_DIV1024;

    // set the counter to overflow at our desired interval
    TC3->COUNT16.COUNT.reg = 0;
    TC3->COUNT16.CC[0].reg = (32768 / 1024) * BLINK_PERIOD_SECONDS;

    // at first boot, cheat the count so that the LED blinks on in a half second
    TC3->COUNT16.COUNT.reg = TC3->COUNT16.CC[0].reg - (32768 / 2048);

    // enable the counter
    TC3->COUNT16.CTRLA.reg |= TC_CTRLA_ENABLE;

    // and enable the interrupt
    TC3->COUNT16.INTENSET.reg = TC_INTENSET_MC(1);
    NVIC_EnableIRQ(TC3_IRQn);
}

//-----------------------------------------------------------------------------
void irq_handler_tc3(void) {
    // if the interrupt flag is set
    if (TC3->COUNT16.INTFLAG.reg & TC_INTFLAG_MC(1)) {
        // clear the interrupt flag
        TC3->COUNT16.INTFLAG.reg = TC_INTFLAG_MC(1);
    }
}

//-----------------------------------------------------------------------------
int main(void) {
    // initialize the clocks
    sys_init();

    // initialize our counter
    timer_init();

    // set up LED output and turn the LED off at first
    HAL_GPIO_LED_out();
    HAL_GPIO_LED_clr();

    // set SLEEPDEEP bit so that __WFI enters standby
    SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;

    while (1) {
        // enter standby mode and stay there until the timer interrupt fires
        __WFI();
        
        // at this point we just woke up! Toggle the LED.
        HAL_GPIO_LED_toggle();

        // and if we just turned on the LED, set the count so that the interrupt
        // fires again in a half-second, turning it back off.
        if (HAL_GPIO_LED_read()) {
            TC3->COUNT16.COUNT.reg = (32768 / 1024) * BLINK_PERIOD_SECONDS - (32768 / 2048);
        }
    }

    return 0;
}