Demo of the usage USBDevice library with Blue Pill STM32F103C8T6 board.

Dependencies:   USBDevice

STM32F103C8T6 USBSerial Demo

This project contains demo of the USB serial usage for a cheap developer board Blue Pill with STM32F103C8T6 mcu.


The USB serial port provides a good communication channel between PC and microcontroller. Especially it can be useful for a debug purposes.


  • by the specifications this board has only 64KB of the flash, but actually it can have 128KB, that will be useful for a debug builds as it requires about 100KB of the flash for this demo.
  • the board can have some problems with an USB because it has wrong value of the pull-up resistor
  • for steady reading of data from a serial port, the project contains python script (it requires PySerial and six python libraries), that is steady to the board reloading
  • the project depends on the fork of the USBDevice library. This fork contains some fixes and support of the BLUE_PILL_STM32F103C8 target.
  • the mbed-os now contains correct code for a clock initialization of the BLUE_PILL_STM32F103C8 target, so you don't need to adjust the board clocks separately

Konstantin Kochin

File content as of revision 0:24604e97c40c:

#!/usr/bin/env python
from __future__ import print_function

import argparse
import sys
from time import sleep

import serial
from import comports
from six import binary_type


def _reset_baud_drate(serial):
    original_baudrate = serial.baudrate
    i = serial.BAUDRATES.index(original_baudrate)
    if i == -1:
        raise ValueError("Unknown baudrate {}".format(original_baudrate))

    if i > 1:
        tmp_baud_rate = serial.BAUDRATES[i - 1]
    elif len(serial.BAUDRATES) > 1:
        tmp_baud_rate = serial.BAUDRATES[i + 1]
        raise ValueError("Only one baudreate is available")

    serial.baudrate = tmp_baud_rate
    serial.baudrate = original_baudrate

def _read_from_serial_device(port, out=sys.stdout, reconnect_on_error=True, repeat_delay=2, **port_params):
    print('Read from the serial port "{}" (use Ctrl+C to exit)'.format(port))
    while True:
            with serial.Serial(port=port, **port_params) as com_port:
                # hack:
                # when you connect usb-com port of the stm32f1 first time, it will work
                # but if you reconnect it, then it stop working, although com port readers doesn't show
                # any error.
                # However I found that it can be solved by 2 ways:
                # - host rebooting (but it's very inconvenient)
                # - changing baudrate. The new value doesn't matter, it should only be different than previous.
                #   Moreover the old values without any problems can be reverted without any problems.
                # TODO: find reason of such strange behavior
                for line in com_port:
                    if isinstance(line, binary_type):
                        line = line.decode('utf-8')

        except serial.SerialException as e:
            print("Error: {}".format(e))
            if not reconnect_on_error:

def _guess_serial_device_name():
    ports = list(comports())
    if len(ports) == 1:
        port = ports[0].device
    elif len(ports) == 0:
        raise ValueError("No serial devices found")
        print("Several serial devices are found. Please specify one of them explicitly:")
        for port_info in ports:
            print("- {}".format(port_info.device))
        raise ValueError("Multiple serial devices are found.")

    return port

def main(args=None):
    parser = argparse.ArgumentParser(prog='serial_reader',
                                     description='Helper program that read text from a serial port. '
                                                 'Unlike other com programs it automatically reconnect '
                                                 'to serial port if any error occurs. It is useful for '
                                                 'microcntrollers debugging when you want to reboot it and'
                                                 'the connection will be lost for some time.')
    parser.add_argument('port', nargs='?', help='Serial port device name. It it is not specifies, then'
                                                'it will be found automatically if it is possible')
    parser.add_argument('--baudrate', default=_DEFAULT_BAUD_RATE,
                        help="baud rate. Default: {}".format(_DEFAULT_BAUD_RATE))
    parsed_args = parser.parse_args(args)

    if parsed_args.port is None:
            port = _guess_serial_device_name()
        except Exception as e:
        port = parsed_args.port

    except KeyboardInterrupt:
        print("Exit ...")

if __name__ == '__main__':