Persistent Arduino `stk500_getsync()` Error via Raspberry Pi Zero W 2 GPIO for Remote Upload

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1

Overview: We're trying to remotely upload Arduino sketches (.ino files) from a Raspberry Pi Zero W 2 to an Arduino Uno using arduino-cli via the Pi's native UART GPIO pins. Despite confirming serial communication and a successful manual reset, automated uploads consistently fail with the avrdude: stk500_getsync() error.

Goal: Achieve reliable, automated remote sketch uploads from the Raspberry Pi to the Arduino without physical intervention (i.e., no manual reset button presses). Acquiring new hardware (e.g., a dedicated USB-to-Serial converter) is currently not an option.

Hardware Details:

  • Raspberry Pi: Pi Zero W 2
    • OS: Raspberry Pi OS (formerly Raspbian) 32-bit lite
    • UART Configuration: Enabled and used on GPIO14 (TXD) and GPIO15 (RXD).
    • GPIO for Reset: GPIO17 (Physical Pin 11) is used for DTR emulation.
  • Arduino Board: Arduino Uno (Standard ATmega328P based)
  • Connection Method: Direct GPIO-to-Arduino serial connection.
  • Components in Reset Circuit:
    • Capacitor: 0.1µF (100nF, labeled "104M") ceramic capacitor. Two different capacitors of this value have been tried.
    • Resistor: 10kΩ pull-up resistor.

Current Wiring Configuration:

  • Serial Communication:
    • Raspberry Pi GPIO14 (TXD) → Arduino RX
    • Raspberry Pi GPIO15 (RXD) → Arduino TX
    • Raspberry Pi GND → Arduino GND (common ground established)
  • Auto-Reset Circuit (Current Configuration):
    • Raspberry Pi GPIO17 (Physical Pin 11) → One lead of 0.1µF Capacitor.
    • The other lead of the 0.1µF Capacitor → Arduino RESET pin.
    • Arduino RESET pin → One lead of 10kΩ Resistor.
    • The other lead of the 10kΩ Resistor → Arduino 5V pin.

Problem Description & Troubleshooting Steps Taken:

  1. Initial stk500_getsync() Error: Uploads failed with this error from the start.
  2. Manual Reset Success: Pressing the Arduino's physical RESET button immediately after initiating the arduino-cli upload command successfully allows the sketch to upload. This confirms:
  • The serial TX/RX wiring is correct.
  • arduino-cli and avrdude are functional.
  • The Arduino's bootloader is functional.
  • The issue is solely with the auto-reset mechanism.
  1. ARDUINO_CLI_SERIAL_DTR_GPIO Attempts: The environment variable export ARDUINO_CLI_SERIAL_DTR_GPIO=17 was used to attempt automated reset via arduino-cli. This consistently resulted in stk500_getsync().
  2. Capacitor & Resistor Configuration:
  • Initially tried with only 0.1µF capacitor.
  • Replaced the 0.1µF capacitor with a new one (same value).
  • Added a 10kΩ pull-up resistor (RESET to 5V) as per standard robust auto-reset circuits.
  • No change in stk500_getsync() errors with these modifications.
  1. gpiozero Programmatic Reset Attempts:
  • A Python script using gpiozero was created to generate a precise LOW pulse on GPIO17 immediately before the arduino-cli upload command.
  • Crucial Observation: When this gpiozero script is run, the Arduino's serial output (from a separate impact_sensor.py program monitoring it) briefly stops and then resumes. This confirms that the Raspberry Pi's GPIO17 pulse is successfully resetting the Arduino.
  • However, even with the confirmed reset, stk500_getsync() persists.
  • Various pulse timings (e.g., 100ms, 10ms, 5ms, 1ms LOW pulse, with varying delays before/after) were tested, all resulting in the same synchronization error.
  1. Serial Port Contention: Confirmed that no other processes (like impact_sensor.py) are actively using the serial port during upload attempts. The issue is not serial port contention.
  2. "Serial Bypass Mode" Symptom: When the full auto-reset circuit is connected, the Arduino does not appear to be constantly resetting, but rather behaving as if it's waiting for serial communication or running its sketch, despite the reset pulse being detected. It does not appear to "stay" in bootloader mode.

Current Hypothesis: The issue likely lies in the extremely tight and precise timing window required by the Arduino's bootloader for avrdude synchronization, which the Raspberry Pi's GPIO, even with external components and programmatic control, is unable to consistently hit. The electrical characteristics of the GPIO pulse might not be sufficiently "clean" or precise compared to a dedicated DTR line from a USB-to-Serial converter.

Constraint: Acquiring new hardware (e.g., a dedicated USB-to-Serial converter) is currently not an option. We need to find a solution using the existing components.

2

Why don't you just use the Pi's USB port?

3

The USB port is broken.

4

So buy a new one and have done.

5

I appreciate your feedback, but I posted this so as to learn how to upload a sketch via serial, not order something on amazon.

I have another one with a functioning micro-USB port, but it will be occupied by another peripheral.

If this is impossible (which it may be), I'd be glad to hear that from someone!

6

Never order anything on Amazon their prices are way way too high.

7

Apparently there is a way to have the DTR signal on a GPIO but I can't find the necessary information.
If I find it I'll tell you.

I add:
See if this helps you.

8

Thanks, I tried for the better part of a day with three different AI tools. I tried a script that tried 130 different combinations of timings and it didn't work. I used the 10k pulldown resistor and requisite capacitor.

Apparently flashing the mega2650 bootloader to optiboot helps because it has more forgiving timing, but I was unable to flash it via SPI from another Arduino Yún... nothing happened when I tried burning the bootloader.

I got a an MPU60 instead of an LISD3DSH accelerometer and am just using a pi with no Arduino now. Would love to be able to come back to this and make it work at some point.