How to debug Arduino Nano 33 IoT code?

The Arduino Nano 33 IoT (ATSAMD21G18, 3.3 V) is great for “real” debugging. Here’s a focused playbook just for this board.

1) Quick wins (native USB board)

• Select the right core/board: Boards Manager → Arduino SAMD Boards → Arduino Nano 33 IoT.

• USB gotcha: a bad sketch can “hide” the port. Double-tap RESET → bootloader mode (LED pulsing) → upload again.

• Serial init (with timeout):

void setup() {
  Serial.begin(115200);
  unsigned long t0 = millis();
  while (!Serial && (millis()-t0 < 1500)) { } // don't hang forever
  Serial.println("Hello from Nano 33 IoT");
}

• 3.3 V only. Don’t feed 5 V signals into pins.

2) Print-style debugging (fastest)

• Use Serial.print/println liberally. For periodic sanity:

static uint32_t last;
if (millis() - last > 1000) { last = millis(); Serial.printf("t=%lu\n", millis()); }

• On SAMD, Serial is the USB CDC port; Serial1 is on D0/D1 if you need an external USB-TTL adapter for out-of-band logs.

3) Real step-debugging over SWD (breakpoints, watch, step)

Hardware you need (any one): Atmel-ICE, J-Link, Black Magic Probe, or CMSIS-DAP probe.
Connection (to the test pads on the underside):

• SWDIO → SWDIO pad

• SWCLK → SWCLK pad

• RESET → RST pad

• VTREF → 3V3 (board’s 3.3 V)

• GND → GND
  (Use pogo pins or carefully solder thin wires; keep them short.)

Arduino IDE 2 steps:

1. Tools → Programmer → your probe (e.g., Atmel-ICE).

2. Click the bug icon (Debug). Choose the probe + Nano 33 IoT.

3. Set breakpoints, use Continue/Step, Watch, Call Stack.

4. If you ever lose the bootloader, Tools → Burn Bootloader (with the probe) restores it.

Tip: For rock-solid sessions, power the board via USB and connect SWD in parallel (don’t let the probe back-power it).

4) Catching crashes & hangs

Minimal HardFault trap (will blink and print if possible):

extern "C" void HardFault_Handler(void){
  pinMode(LED_BUILTIN, OUTPUT);
  for(;;){ digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN)); delay(150); }
}

Add timeouts to all loops waiting for hardware; never rely on infinite waits.

5) I²C/SPI/UART sanity (common culprits)

• I²C pull-ups: 4.7 kΩ to 3.3 V typically. Slow down first: Wire.setClock(100000);

• Bus lockups: Wire.setWireTimeout(25000, true); to auto-recover.

• I²C scanner to verify addresses:

#include <Wire.h>
void setup(){ Serial.begin(115200); Wire.begin(); }
void loop(){
  for (uint8_t a=1; a<127; a++){ Wire.beginTransmission(a); if(!Wire.endTransmission()) Serial.printf("0x%02X\n",a); }
  delay(2000);
}

• SPI: Check CS polarity and that only one device is selected at a time.

• UART: Use Serial1 on D0/D1; match baud on both sides.

6) Wi-Fi (NINA-W102) troubleshooting

• Update NINA firmware: Tools → WiFi101 / WiFiNINA Firmware Updater → “Test Connection” → “Update Firmware” to the version suggested by your WiFiNINA library.

• Basic link test:

#include <WiFiNINA.h>
void setup(){
  Serial.begin(115200);
  WiFi.disconnect(); delay(200);
  Serial.print("FW: "); Serial.println(WiFi.firmwareVersion());
  int s = WiFi.scanNetworks();
  Serial.printf("Found %d networks\n", s);
}
void loop(){}

• If WL_NO_MODULE, the NINA module isn’t answering → re-seat USB, update firmware, or check 3.3 V rail.

7) Timing & non-blocking pattern

Replace long delay() calls with the millis() pattern so your code stays responsive:

uint32_t tBlink=0;
void loop(){
  if (millis()-tBlink > 500){ tBlink = millis(); digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN)); }
  // …other work…
}

8) Memory & strings on SAMD

• SAMD21 has much more RAM than AVR, but still avoid huge dynamic String churn in tight loops. Prefer fixed buffers + snprintf.

• Enable all compiler warnings (IDE Preferences) and fix them; many bugs surface there.

9) A quick “board health” checklist

• Port shows up; double-tap reset enters bootloader if not

• Serial prints at 115200 with a 1.5 s timeout

• I²C devices detected by scanner; add/verify pull-ups

• WiFiNINA firmware matches library; scan works

• For tricky bugs, attach SWD probe and step through