🛠 Diagnostic AT du module et version de firmware

Objectives

Troubleshoot the cellular part at a low level: send AT commands to the u-blox SARA-U201 module to read signal strength, the registered operator, the module's firmware version, and check the SIM status. When MKRGSM fails in a loop without a clear message, the AT commands tell you everything: missing SIM, blocked PIN, network not found, rejected APN.

Hardware

• Arduino MKR GSM 1400 board + antenna + LiPo + SIM without PIN
• Arduino SAMD core + MKRGSM library (for sendTestRequest, which serializes AT commands without directly controlling the UART dedicated to the modem)
• Note: the modem is on a dedicated UART (SerialGSM/Serial2, SERCOM4 with RTS/CTS flow control, pins 26-29 of the variant); the simplest approach is to use the library rather than bypassing this port.

Complete Code

#include <MKRGSM.h>
#include "arduino_secrets.h"
const char PINNUMBER[] = SECRET_PINNUMBER;

GSM gsmAccess;

void at(const char* cmd) {
  Serial.print(">> "); Serial.println(cmd);
  char rep[128] = {0};
  // sendTestRequest envoie la chaine brute au module et recupere la reponse.
  gsmAccess.sendTestRequest(cmd, rep, sizeof(rep));
  Serial.print("<< "); Serial.println(rep);
}

void setup {
  Serial.begin(9600);
  while (!Serial) {}

  at("AT");            // vie du module : doit repondre OK
  at("AT+CGMR");       // version du firmware du module
  at("AT+CPIN?");      // etat SIM : READY, SIM PIN, ou SIM NOT INSERTED
  at("AT+CSQ");        // signal : 0..31,99 ; 99 = aucun
  at("AT+COPS?");      // operateur enregistre et mode (0 = pas enregistre)
  at("AT+CGREG?");    // etat d'enregistrement GPRS
  at("AT+CGDCONT?");   // APN actif

  // Puis tentative normale de demarrage pour croiser les indices
  int r = gsmAccess.begin(PINNUMBER);
  Serial.print("gsmAccess.begin = "); Serial.println(r);
}

void loop {}

Expected Verification

AT responds OK; AT+CPIN? returns READY (otherwise: SIM poorly inserted, or PIN needs unlocking with AT+CPIN="1234"); AT+CSQ gives a reasonable value between 0-31 (99 = no antenna/no network); AT+COPS? displays the operator; AT+CGMR displays the firmware version of the SARA-U201, to be noted in your maintenance log. Cross-reference with the return code of begin: 1 = SIM not initialized, 2 = network not found, 3 = GPRS error (see MKRGSM).

Common Errors

• Sending AT commands in parallel with MKRGSM: the module is controlled by the library, raw commands outside its context corrupt the state; do everything via sendTestRequest or restart everything.
• >
• Confusing the codes: AT+CSQ also returns the bit error rate; only the first number counts for signal strength.
• PIN permanently blocked (PUK) due to repeated incorrect AT+CPIN attempts: unlock with the PUK from a phone, not from the board.
• Truncated multi-line responses: ensure a sufficiently large response buffer and look for "OK"/"ERROR" at the end of the response.
• 2G being phased out: a desperately empty AT+COPS? may simply mean that the operator has shut down the band used by the board.