?? strange serial behaviour
I added one more line to the code above … just suspend loop
and now serial monitor is printing every 3 seconds. It looks like the XIAO is firing the softwaretimer every 1 second but serial is sending output every 3 seconds. Strange !?
I still can not check if xiao is really “system-on sleeping” (and consuming less power) hence I don’t have precise meter. If not - I will try tu use hardware timer.
If someone could check this out …would be great 
#include <Arduino.h>
#include <Adafruit_TinyUSB.h> // for Serial
#include <bluefruit.h>
/* SoftwareTimer is a helper class that uses FreeRTOS software timer
* to invoke callback. Its periodic timing is flexible as opposed to
* hardware timer and cannot be faster than rtos's tick which is configured
* at ~1 ms interval.
*
* If you need an strict interval timing, or faster frequency, check out
* the hw_systick sketch example that use hardware systick timer.
*
* http://www.freertos.org/RTOS-software-timer.html
*/
SoftwareTimer blinkTimer;
void setup()
{
Serial.begin(115200);
Bluefruit.begin(); // Sleep functions need the softdevice to be active.
sd_power_mode_set(NRF_POWER_MODE_LOWPWR);
pinMode(LED_BUILTIN, OUTPUT); // initialize digital pin LED_BUILTIN as an output.
blinkTimer.begin(1000, blink_timer_callback); // Configure the timer with 1000 ms interval, with our callback
blinkTimer.start(); // Start the timer
suspendLoop(); // Suspend Loop() to save power, since we didn't have any code there
}
void loop()
{
// do nothing here , loop is already suspended, CPU will not run loop() at all
}
/**
* Software Timer callback is invoked via a built-in FreeRTOS thread with
* minimal stack size. Therefore it should be as simple as possible. If
* a periodically heavy task is needed, please use Scheduler.startLoop() to
* create a dedicated task for it.
*
* More information http://www.freertos.org/RTOS-software-timer.html
*/
void blink_timer_callback(TimerHandle_t xTimerID)
{
// freeRTOS timer ID, ignored if not used
(void) xTimerID;
Serial.print("timer callback at - ");
Serial.println(millis());
digitalToggle(LED_BUILTIN);
sd_app_evt_wait(); // puts the nrf52 to sleep when there is nothing to do
}
serial monitor output
...
13:40:50.928 -> timer callback at - 222418
13:40:50.928 -> timer callback at - 223418
13:40:50.928 -> timer callback at - 224418
13:40:53.951 -> timer callback at - 225418
13:40:53.951 -> timer callback at - 226418
13:40:53.951 -> timer callback at - 227418
13:40:56.952 -> timer callback at - 228418
13:40:56.952 -> timer callback at - 229418
13:40:56.952 -> timer callback at - 230418
...