Robin Nano v1.2 + Raspberry Pi - Intermittent USB communication timeout / print stall during long prints

General Discussion
1

Basic Information:

  • Printer: Two Trees Sapphire Plus (SP-5, CoreXY)

  • MCU / Board: MKS Robin Nano v1.2 (STM32F103)

  • Host: Raspberry Pi (aarch64, Debian GNU/Linux 13 Trixie)

  • Klipper: v0.13.0-573-g644cda5e

  • Connection: USB (usb-1a86_USB_Serial-if00-port0)

Problem Description:

During long prints, Klipper intermittently loses communication with MCU and stops printing. The issue occurs randomly - sometimes after 10 minutes, sometimes after 90% of a long print.

Symptoms in klippy.log:

Pattern 1 - Classic timeout:

bytes_retransmit increasing → rto=0.200 → 0.800 → 1.600 → 3.200
Timeout with MCU 'mcu'
Transition to shutdown state: Lost communication with MCU 'mcu'

Pattern 2 - Print stall (most recent):

print_stall=1
print_time frozen (not advancing)
bytes_retransmit=6,987,000+ (accumulated over entire print)
rto=5.000
send_seq=2634251, receive_seq=2634248 (3 packets lost, never recovered)

What I have tried:

  1. Changed USB port on Raspberry Pi → same issue

  2. Replaced USB cable (USB-A to USB-B printer cable) → same issue

  3. USB autosuspend disabled via udev rule:

ACTION=="add", SUBSYSTEM=="usb", TEST=="power/control", ATTR{power/control}="on"

-> confirmed all devices showing on 4. Checked RPi voltage - vcgencmd get_throttled returns 0x0 (no undervoltage) 5. RPi temperature - 47~50°C during printing (heatsink + fan installed) 6. DSI display removed - issue still occurs 7. Moved USB cable away from stepper motor cables - issue still occurs

Key observations:

  • Issue started after adding a DSI touchscreen display, but removing the display did not resolve it

  • bytes_retransmit accumulates significantly during long prints, suggesting ongoing instability throughout the print, not just at the moment of failure

  • RPi CPU load is normal (sysload ~0.1~1.0), temperature is acceptable

  • No bytes_invalid recorded

  • rpi MCU (Linux host MCU) communication is completely stable throughout - only the main mcu (Robin Nano) connection fails

Question:

Is this a known issue with Robin Nano v1.2 USB communication? Would switching to UART (USART1 via PA9/PA10) resolve this? Any other suggestions?

klippy (2).zip (1.2 MB)

2

Not really applicable as this communication route is not on the USB hardware.

The different error messages makes me lean towards a Pi hardware problem.

This is not even an educated guess BUT your symptoms are reminiscent of a cracked solder ball under a BGA chip. When you attached the touchscreen was the Pi exposed to any drops or handling that could have flexed the mainboard?

Perhaps a CLI stress test with USB unplugged would uncover an issue.

If you had a second printer and/or RPi you could swap things around and figure out which component is bad.

Hopefully someone with more “hands on” RPi experience will chime in soon.