steami: USB disconnect during large file transfers via mpremote.

[nedseb]

Description

When copying large files to the STM32WB55 MicroPython filesystem via mpremote cp, the USB connection intermittently drops mid-transfer. The STM32F103 DAPLink interface resets, causing a SerialException: device reports readiness to read but returned no data.

Symptoms

• mpremote cp fails with SerialException or OSError: [Errno 5] Input/output error
• The board disappears from /dev/ttyACM*
• Happens intermittently — not every time, suggesting a race condition
• More likely with larger files (>1 KB)
• Small files copy reliably

Suspected cause

Race condition in the DAPLink USB CDC serial bridge. The STM32F103 handles USB CDC traffic between the host and the STM32WB55. During large transfers, the I2C command handler or the 30ms hook may interfere with the USB CDC data flow, causing a buffer overflow or missed USB packet that triggers a USB reset.

Steps to reproduce

# Copy a ~2 KB Python file to the board
mpremote cp large_script.py :main.py
# Repeat several times — fails intermittently

Environment

• DAPLink firmware: release_letssteam + config zone commands (PR steami: Add config zone in F103 internal flash gap #4)
• Host: Linux, mpremote via USB CDC

Related

• bug: USB disconnect during large file transfers via mpremote. micropython-steami-lib#244 — Same issue tracked from the MicroPython side
• steami: Stabilize config zone firmware (I2C IRQ race condition). #5 — I2C IRQ race condition investigation