The common way to save power with microcontroller is to go to sleep and use the watchdog to wakeup. Indeed, most of the power are drain while the microcontroller is doing nothing (i.e. waiting between two sample). This will allow you to go down to 6mA at the microcontroller level while sleeping (if powered in 3.3V).
The very popular Low-Power library is supporting the SAMD21G but only after making some patching. A good alternative is to use the Adafruit SleepyDog Library.
To install the library in the Arduino IDE, go in the menu Tools -> Manage Libraries... In the library manager, search for Sleepy and install the Adafruit SleepyDog Library by Adafruit.
The usage is quite simple:
#include <Adafruit_SleepyDog.h>
void setup() {
pinMode(LED_BUILTIN, OUTPUT);
digitalWrite(LED_BUILTIN, HIGH);
delay(16000);
}
void loop() {
digitalWrite(LED_BUILTIN, LOW); // Show we're sleeping
// Sleep
Watchdog.sleep();
digitalWrite(LED_BUILTIN, HIGH); // Show we're awake again
delay(5000);
}When you're calling the function Watchdog.sleep(), the board will be idle for 16 seconds and the consumption is going as low as 6mA when powered at 3.3V (which is quite better than 18mA with the BareMinimum program).
The function Watchdog.sleep() sets the MCU in STANDBY mode. According to the datasheet of the SAMD21, the STANDBY mode is the lowest power consumption mode available on a SAMD21 MCU. In STANDBY mode, all system clocks are disabled and the voltage regulators are set to run in low power mode.
The only trick is when you want to upload a new program on your board... When the board is idle, you cannot upload a new program on it because it is not listening to the USB serial. So, never upload a program without some activities (real activities or fake activities like the delay(5000) above) to have the time slot to upload successfully.
Useful resources: