🛠 Sommeil bas courant du module et réveil

Objectives

Extend battery autonomy for a field station: put the SARA-U201 module into reset (the SAMD core already keeps the modem in reset at startup to save energy), put the SAMD21 to sleep, and wake the board on an event — button press or periodic alarm — for a quick measurement and transmission. The Arduino design documents a drop from "several mA to a few µA" when the modem and crypto chip are powered off.

Hardware

• Arduino MKR GSM 1400 board + LiPo (the only power source for the test)
• Push button on D2 to ground (wake-up)
• ArduinoLowPower library (SAMD core); MKRGSM for the transmission phase
• A multimeter set to ammeter mode to verify actual power consumption

Complete code

#include <ArduinoLowPower.h>

const uint8_t BTN_REVEIL = 2;
volatile bool reveille = false;

void onReveil { reveille = true; }

void setup {
  // 1) Maintenir le modem GSM en reset pendant le sommeil.
  //    GSM_RESETN (broche 31 de la variante) est actif a l'etat BAS.
  pinMode(GSM_RESETN, OUTPUT);
  digitalWrite(GSM_RESETN, LOW);

  // 2) USB inutile en sommeil profond : couper la CDC libere de l'energie.
  Serial.begin(9600);
  delay(3000);                      // laisser le temps d'ouvrir la console
  Serial.println("Entree en sommeil, appuyer sur D2");

  LowPower.attachInterruptWakeup(BTN_REVEIL, onReveil, FALLING);
}

void loop {
  reveille = false;
  LowPower.sleep;                 // SAMD en mode attente, reveil par IT

  // --- Reveil : phase active courte ---
  digitalWrite(GSM_RESETN, HIGH);   // relancer le modem
  delay(1000);
  Serial.begin(9600);
  Serial.println("Reveille ! Mesure + transmission ici.");
  // ... mesure capteur, gsmAccess.begin, envoi GPRS/SMS, shutdown ...;
  delay(3000);
  Serial.end;
  digitalWrite(GSM_RESETN, LOW);    // remettre le modem en reset
}

Expected verification

On the multimeter, the average current drops significantly when LowPower.sleep is reached and the modem is held in reset; pressing D2 wakes the board and the console displays the wake-up message. Measure the average over several minutes: a registered module transmits in bursts (paging), and the BQ24195L charging PMIC can mask microamperes if the charger is powered via USB.

Common errors

• Believing the module is "off" because the sketch doesn't call it: without held reset or gsmAccess.shutdown, it remains registered to the network and consumes power during permanent paging.
• >
• Wake-up failure: attachInterruptWakeup must target a pin capable of interrupts; test the pinout before sealing the enclosure.
• Forgetting to restart the modem after wake-up: GSM.begin fails if GSM_RESETN is not released.
• Relying on the USB clock: during sleep, millis does not advance in the same way; for precise periodic wake-ups, use the RTC (supported by the SAMD core) rather than successive sleep calls.
• LiPo not fully charged: below 3.4 V, radio peaks cut off the power supply and corrupt the autonomy test.