🛠 Diagnostic AT et qualité de signal

Objectives

Troubleshoot a cellular link using AT commands on the SARA-R410M module: registration status (AT+CEREG?), radio quality (AT+CSQ), serving cell and technology (AT+QENG). The MKRGSM library exposes sendModemCommand to send raw AT commands without rewriting a UART driver. The modem is on a dedicated UART (SERCOM4: GSM_TX PA12, GSM_RX PA13, RTS/CTS PA14/PA15) whose flow control is not enabled by the core — hence truncated responses under heavy AT traffic.

Hardware

• 1 MKR NB 1500 board + antenna + data SIM
• Micro-USB cable + serial console
• MKRGSM library (AT probe commands are available in it)

Full code

// Diagnostic AT periodique — MKR NB 1500
#include <MKRGSM.h>
#include "arduino_secrets.h"

const char PINNUMBER[] = SECRET_PINNUMBER;
GSM gsm;

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

  bool ok = false;
  while (!ok) {
    if (gsm.begin(PINNUMBER) == GSM_READY) ok = true;
    else delay(3000);
  }
}

void loop {
  Serial.println(F("--- Diagnostic ---"));
  Serial.println(F("Enregistrement :"));
  gsm.sendModemCommand("AT+CEREG?", &Serial);   // 0,1 = enregistre, 0,5 = en roaming
  Serial.println(F("Qualite signal :"));
  gsm.sendModemCommand("AT+CSQ", &Serial);      // +CSQ: rssi,ber
  Serial.println(F("Cellule/RAT :"));
  gsm.sendModemCommand("AT+QENG=\"servingcell\"", &Serial);
  delay(30000);
}

Expected verification

AT+CEREG? responds with +CEREG: 0,1 (registered) or 0,5 (roaming). AT+CSQ responds with +CSQ: 20,99: the first number is 0–31 (RSSI, dBm ≈ -113 + 2×CSQ, so 20 ≈ -73 dBm, excellent; < 10 ≈ -93 dBm, borderline), 99 = unknown. AT+QENG indicates the RAT (NB-IoT or LTE-M) and the band — verify that these are the expected ones.

Common errors

• Interpreting CSQ as a percentage: it is a logarithmic scale from 0–31; 99 means "no measurement", not "zero signal".
• Incomplete or corrupted AT responses: the modem's UART does not use RTS/CTS (open core bug); limit the volume of AT commands and add pauses.
• The module freezes after a software restart (forum thread "SARA Modem freezes when restarted"): prefer a clean shutdown (shutdown) or cutting power over an AT restart.
• sendModemCommand without output stream: make sure to pass &Serial to see the responses.
• In case of persistent failure, the historical reliability of MKRGSM is known (issue #66): cross-check with the MKRNB library before declaring the board dead.