Blue Pill: USB CDC hangs (LED stops) when PC sends 193+ bytes at once, 192 bytes works. Known issue?

[yyoshida01234-create]

Hi,

I'm seeing a hang on a Blue Pill when receiving more than 192 bytes over USB CDC, and I'd like to know if this is a known problem or if I'm missing something.

Symptom

When the PC sends 193 bytes or more in one burst, the MCU stops completely (the LED blink in loop() stops) and only recovers after reset / re-plug.
192 bytes or fewer work fine.
Since 192 = 64 × 3, I suspect it is related to the CDC receive queue size.

Bytes sent	Result
65	OK
129	OK
192	OK
193	Hang
256	Hang

Environment

• Board: Blue Pill F103C8 [genuine]
• Arduino_Core_STM32: 2.10.1
• IDE: Arduino IDE 2.3.11
• Upload method: Maple DFU Bootloader 2.0
• USB support: CDC (generic 'Serial' supersede U(S)ART)
• U(S)ART support: Enabled (generic 'Serial')
• USB speed: Low/Full Speed
• Optimize: Smallest (-Os), Newlib Nano, no debug symbols/core logs
• Host: [Windows 10/11], Tera Term [version]

Sketch (USB CDC <-> USART2 bridge)

#include <Arduino.h>

// USART2 RX=PA3, TX=PA2
HardwareSerial uart2(PA3, PA2);

const int LED = PB2;
uint32_t lastBlink = 0;

void setup() {
  pinMode(LED, OUTPUT);
  digitalWrite(LED, LOW);

  Serial.begin(115200);   // USB CDC
  SerialUSB.dtr(false);
  Serial.dtr(false);

  uart2.begin(115200, SERIAL_8N1);
}

void loop() {
  if (millis() - lastBlink >= 250) {
    lastBlink = millis();
    digitalWrite(LED, !digitalRead(LED));
  }

  // USB -> UART
  while (Serial.available() > 0 && uart2.availableForWrite() > 0) {
    uart2.write((uint8_t)Serial.read());
  }

  // UART -> USB
  while (uart2.available() > 0 && Serial.availableForWrite() > 0) {
    Serial.write((uint8_t)uart2.read());
  }
}

Nothing in loop() blocks, so I suspect the problem is in the USB CDC layer rather than the sketch.

Additional tests

• Read-and-discard only sketch (no USART2): [NG]
• USART2 baud raised to 921600: [NG]
• CDC_OUT_EP changed from PCD_DBL_BUF to PCD_SNG_BUF in usbd_ep_conf.c: [NG]

What I suspect

The USART side is slower than USB, so the 192-byte receive queue fills up, and the hang seems to occur when reception is resumed after the sketch reads data. This looks similar to #804 (OUT endpoint double buffer getting out of sync after NAKs), but I haven't confirmed it is the same cause.

Questions

1. Is this a known problem in 2.10.1 (or related to CDC_ReceiveQueue fails after CPU halt #804)?
2. Is there a recommended workaround, e.g. the PCD_SNG_BUF change or a larger CDC_RECEIVE_QUEUE_BUFFER_SIZE?
3. Is there anything else I should check (e.g. getting a HardFault backtrace) before filing an issue?

Thanks!

[fpistm]

I need to work on the USB for the next release. It is a know issue.