Fixes hanging on sleep_ms() after deep sleep

[gregyedlik]

As documented in this repo for example: https://github.com/ghubcoder/PicoSleepDemo

[gregyedlik]

A simple modified hello_sleep.c to reproduce the original problem and test this fix:

/**
 * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include <stdio.h>
#include "pico/stdlib.h"
#include "pico/sleep.h"

#include "hardware/rtc.h"

static bool awake;

static void sleep_callback(void) {
    printf("RTC woke us up\n");
    awake = true;
}

static void rtc_sleep(void) {
    // Start on Friday 5th of June 2020 15:45:00
    datetime_t t = {
            .year  = 2020,
            .month = 06,
            .day   = 05,
            .dotw  = 5, // 0 is Sunday, so 5 is Friday
            .hour  = 15,
            .min   = 45,
            .sec   = 00
    };

    // Alarm 10 seconds later
    datetime_t t_alarm = {
            .year  = 2020,
            .month = 06,
            .day   = 05,
            .dotw  = 5, // 0 is Sunday, so 5 is Friday
            .hour  = 15,
            .min   = 45,
            .sec   = 10
    };

    // Start the RTC
    rtc_init();
    rtc_set_datetime(&t);

    printf("Sleeping for 10 seconds\n");
    uart_default_tx_wait_blocking();

    sleep_goto_sleep_until(&t_alarm, &sleep_callback);
}

int main() {
    while(1)
    {
        stdio_init_all();
        printf("Hello Sleep!\n");

        printf("Switching to XOSC\n");

        // Wait for the fifo to be drained so we get reliable output
        uart_default_tx_wait_blocking();

        // UART will be reconfigured by sleep_run_from_xosc
        sleep_run_from_xosc();

        printf("Switched to XOSC\n");

        awake = false;

        rtc_sleep();

        // Make sure we don't wake
        while (!awake) {
            printf("Should be sleeping\n");
        }

        printf("Gonna go sleep_ms for 5 sec\n");
        sleep_ms(5000);
        printf("Done, restart!\n");
    }
}