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Upload error · bossac

Fix bossac “No device found on COM5” (Uno R4 WiFi, Nano 33 IoT, MKR, Zero)

bossac couldn't reach the SAMD/RA4M1 bootloader. Double-tap RESET to enter it, re-select the new port, and close anything holding the serial port.

Arduino Uno R4 WiFiArduino Nano 33 IoTArduino MKR WiFi 1010Arduino ZeroXIAO SAMD21Updated 3 Nov 2026

Community draft — report a mistake

What you see

OutputOutput
Performing 1200-bps touch reset on serial port COM5
Waiting for upload port...
No upload port found, using COM5 as fallback
"C:\Users\me\AppData\Local\Arduino15\packages\arduino\tools\bossac\1.9.1-arduino5/bossac" -d --port=COM5 -U -i -e -w "C:\...\sketch.ino.bin" -R
Set binary mode
No device found on COM5
Send auto-baud
Set binary mode
Failed uploading: uploading error: exit status 1

bossac uploads to boards whose USB port is run by the sketch itself: SAMD21/SAMD51 boards like the Nano 33 IoT, MKR and XIAO SAMD21, and the Uno R4 WiFi through its USB bridge. The IDE resets the board into its bootloader with a 1200-baud “touch”. When that fails, bossac finds nobody on the port.

Double-tapping RESET enters the bootloader by hand and works even when the sketch has crashed.

Quick fix — try in order

  1. 1

    Double-tap RESET

    Press RESET twice quickly. The built-in LED pulses (SAMD boards) to show the bootloader is waiting. Re-select the port — it may have a new number — and upload.

  2. 2

    Close Serial Monitor and other serial apps

    The 1200-baud reset trick fails if another program holds the port.

  3. 3

    Fix the sketch that crashes USB

    On these boards USB runs inside your sketch. A crash, a hard fault or a sleep mode right at startup removes the port. After a double-tap, upload Blink first.

  4. 4

    Update the board package

    Boards Manager → Arduino SAMD Boards / Arduino UNO R4 Boards → latest. It also reinstalls the Windows driver.

Why it happens

How does the reset normally work?

The IDE opens the port at 1200 baud and closes it; the sketch's USB code sees that and reboots into the bootloader, which appears as a new port. If the sketch isn't running properly, nothing happens and bossac talks to an empty port.

Uno R4 WiFi specifics

Its USB goes through an ESP32-S3 bridge. If the bridge firmware is outdated, uploads fail with this message; update it with the Arduino “UNO R4 WiFi Firmware Updater” tool.

Board-specific notes

XIAO SAMD21: No RESET button: short the two RST pads twice quickly with tweezers. The orange LED fades when the bootloader is active.

Arduino Uno R4 Minima: Uses dfu-util, not bossac — see No DFU capable USB device.

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