STM32F103C8T6 - USB CDC Serial port can send to USB Host but not receive

[housey2k]

Describe the bug
When using the Serial monitor on Arduino IDE or any other software like PuTTY the STM32 will successfully send data to the computer, but it fails to receive and consequently reply

To Reproduce
This is the code I am using:

void setup() {
  // Initialize USB Serial
  Serial.begin(115200);
  while (!Serial) {
  }

  Serial.println("Echo");
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();   // Read a character
    Serial.print(c);          // Echo it back
  }
}

Steps to reproduce the behavior:
1 - Flash code
2 - Receive the message "Echo"
3 - When typing something in the serial monitor and sending, the board should reply with what was typed

**Expected behavior:
Receive a response from the board through USB

Desktop (please complete the following information):

• OS: Windows 11
• Arduino IDE version: 2.3.6
• STM32 core version: 2.11.0
• Upload method: SWD

Board:

• Name: STM32 Bluepill
• Extra hardware used if any: ST-Link V2

[fpistm]

Hi @housey2k
Is it a bluepill board? Is it a legacy stm32?

[housey2k]

Hi @housey2k Is it a bluepill board? Is it a legacy stm32?

STM32 Bluepill, as stated in the post
Something interesting is that when using my own code on STM32CubeIDE or using Roger's STM32Duino core, the duplex data transmission works perfectly. For me this is either an actual glitch, or a virtual limitation against non genuine chips enforced by ST

[danielmachado99]

I have the same issue. It works perfectly fine with Roger's STM32Duino core. Please let me know if you find any workaround.

[fpistm]

I've tested with a BluePill STM32F103C8 and your sketch works as expected:

So, I guess it is not a genuine STM32.