bluepill-serial-monster
bluepill-serial-monster is a firmware for STM32 Blue Pill that turns it into a 3 Port USB-to-Serial adapter. The firmware implements a USB 2.0 full-speed composite device that consists of 3 USB CDC devices.
STM32 Blue Pill is a ridiculously cheap STM32 development board which is available in many stores around the globe. The board contains decent hardware that supports USB 2.0 Full-Speed, has 3 independent USARTs and enough processing power to handle high-speed UART communications.
Note: some Chinese-made Blue Pill boards have an incorrect pull-up resistor soldered to the USB D+ line (PA12) which prevents them from being successfully detected as a USB device by the host. There is an existing software workaround for this issue, but it is not reliable.
This firmware does not contain the workaround for faulty Blue Pill boards and requires the incorrect resistor to be replaced with the right one. Please refer to the section Fixing USB on Blue Pill Boards for more information.
Some USB controllers work fine even with faulty Blue Pill boards. If your board appears to be OK with your computer, don't bother fixing it.
Features
- 3 independent UART ports;
- Hardware flow control (RTS/CTS) support1;
- DSR/DTR/DCD signals support;
- 7 or 8 bit word length;
- None, even, odd parity;
- 1, 1.5, and 2 stop bits;
- Works with CDC Class drives on Linux, OS X, and Windows;
- Supports all standard baud rates;
- Supports non-standard baud rates2;
- DMA RX/TX for high-speed communications;
- IDLE line detection for short response time;
- No external dependencies other than CMSIS;
(1) UART1 does not support hardware flow control because RTS/CTS pins (PA12, PA11) are used for USB communication and cannot be remapped. If you need hardware flow control, use UART2 or UART3.
(2) As long as your CDC driver does not hesitate to ask.
UART Signal Levels
Although STM32F103C8T6 installed on the Blue Pill board is a 3.3 V device, a number of its inputs are actually 5 V tolerant. This means you can safely use the selected inputs with 3.3 and 5 V TTL devices.
Do not use non 5 V tolerant inputs with 5 V devices as doing that will result in permanent damage of MCU inputs or MCU itself.
When configured as an output, none of the STM32F103C8T6 pins is 5 V tolerant. Make sure you don't accidentally get more than 4.0 V on such pin or damage may occur.
5 V tolerant pins are shown in bold in the next section.
UART Pinout
| Signal | Direction | UART1 | UART2 | UART3 |
|---|---|---|---|---|
| RX | IN | PA10 | PA3 | PB11 |
| TX | OUT | PA9 | PA2 | PB10 |
| RTS | OUT | N/A | PA1 | PB14 |
| CTS | IN | N/A | PA0 | PB13 |
| DSR | IN | PB7 | PB4 | PB6 |
| DTR | OUT | PA4 | PA5 | PA6 |
| DCD | IN | PB15 | PB8 | PB9 |
Note: 5 V tolerant input pins are shown in bold.
Control Signals
RTS, CTS, DSR, DTR, DCD are active-low signals.
DSR, DTR, and DCD are connected to the internal weak pull-up resistors, so they remain inactive at rest.
CTS is pulled down internally, which enables UART TX when nothing is connected to CTS. Hardware flow control is always on, but it does not get in the way of communications as long as nothing is connected to the flow control lines.
RTS can be controlled by the host, but as soon as the UART RX buffer is half-full, RTS is forced to the inactive state. As long as more than one half of the buffer space is available, RTS remains in the state set by the host. Please take this behaviour into account if you rely on the RTS signal to control non-standard periphery.
DSR and DCD are polled 50 times per second.
UART DMA RX/TX buffer size is 1024 bytes.
Flashing Firmware
Download binary firmware from the Releases page.
Flash with ST-LINK or similar programmer.
st-flash --format ihex write bluepill-serial-monster.hex
You can also flash STM32F103C8T6 via a built-in serial bootloader. Visit https://www.st.com/en/development-tools/flasher-stm32.html for instructions and software.
Fixing USB on Blue Pill Boards
STM32 Blue Pill boards come in slightly different variations. Nevertheless, their schematic is very similar. Below you will find the instructions on how to identify and replace the incorrect USB pull-up resistor on any Blue Pill board.
With a digital multimeter, measure the resistance between PA12 and 3.3 V pads on the board. If the resistance reads close to 1.5k (1500 ohms), then your board is either non-faulty or faulty for some other reason, and this section does not apply.
If the resistance if far away from 1.5k (such as 4.7k or 10k), you will have to locate the incorrect resistor on the board and replace it with a 1.5k or 1.8k resistor.
If your board has component names on it, locate R10. Otherwise, trace the board to see where the incorrect resistor is located.
Once you identified the incorrect resistor, replace it with a 1.5k or 1.8k resistor.
Building Firmware
Prerequisites
Install the following software:
Here is an example assuming everything is installed in ~/stm32/, and we use bash:
ARM toolchain and st-link must be added to PATH.
# add ARM toolchain path
export PATH=~/stm32/gcc-arm-none-eabi/bin:$PATH
# add stlink path
export PATH=~/stm32/stlink-install/bin:$PATH
Path to STM32CubeF1 should be also exported (use ~/.bash_profile):
# export STM32Cube
export STM32CUBE_PATH=~/stm32/stm32cube
Building
To build the firmware, cd to the project directory and run
make
To flash the MCU using st-link, run
make flash
To remove object and dependency files, run
make clean
To remove object, dependency, and firmware files, run
make distclean
