STM32F1xx micro-controller C firmware template
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This is the firmware for the sound lever enforcer. It will cut the power of speakers when it's too loud.



The sound level enforcer receives measurements from a sound level meter over Bluetooth. The sound level is then show on a display (0 if it is not connected or receives no data). If this level is below the threshold configured, the display is dim. If the level is above the threshold configured, the brightness is higher. If the level is above the threshold configured, for a set amount of time, a relay it activated. The relay's purpose is to cut the power of speakers/mixer. "too loud" will be shown on the display, and the button will light up. Press the button to reset the device. The relay will be deactivated, and the cycle starts again.

When powered up or reset, the display will show the sound level threshold (followed by the unit "dBa"), and duration (followed by the unit "dBa"). To change the sound level threshold, enter "threshold xxx" on the serial interface (over USB, or UART). To change the sound level duration, enter "duration xxx" on the serial interface (over USB, or UART).


The firmware runs on a black pill development board. It is based on a STM32F103C8T6 micro-controller. This will do all the processing and control the peripherals.

The Bluetooth module is a HC-05. It should be configured to automatically connect to the sound level meter. It should receive the sound level over Bluetooth in the format "123.4 dBa\n". The measurement will be forwarded to the micro-controller over the UART part configured at 115200 bps 8N1.

This does not use a Bluetooth Low Energy (BLE) module because it does not fit the needs. There is no need to save energy since the device needs permanent power just for the display. There is a constant stream of data (not fitting BLE principles). The is a specified Classic Bluetooth profile for serial data: Serial Port Profile (SPP). BLE module use non-standard GATT characteristics for serial data transfer.

The display is a 4-digit 7-segment LED display. This is controlled by a TM1367.

The relay is a SRD-05VDC-SL-C module (250V 10A). Connect it to the device using a 2.5 mm jack.

The momentary button should have a built-in LED.


Connect the peripherals as described below.

HC-05 Bluetooth SPP module:

  • STATE: no connect
  • RX: no connect
  • TX: USART3_RX/PB11
  • GND: ground
  • VCC: 5V
  • EN: no connect

button (momentary, with LED)

  • +: 5V
  • -: 470 Ohm resistor to PB12 (sometimes the resistor is already built in the button)
  • S: GND
  • S: NRST

7-segment display TM1637:

  • GND: ground
  • VCC: 5V
  • DIO: PB13
  • CLK: PB14

relay, connected to 2.5 mm TRS jack:

  • tip, VCC: 5V
  • ring, IN: PB15
  • sleeve, GND: ground

All pins are configured using defines in the corresponding source code.



The source code uses the libopencm3 library. The projects is already a git submodules. It will be initialized when compiling the firmware. Alternatively you can run once: git submodule init and git submodule update.


To compile the firmware run rake.


To generate doxygen documentation run rake doc.


There are two firmware images: bootloader and application. The bootloader image allows to flash the application over USB using the DFU protocol. The bootloader is started first and immediately jumps to the application if it is valid and the DFU mode is not forced (i.e. by pressing the user button on the board or requesting a DFU detach in the application). The application image is the main application and is implemented in application.c. It is up to the application to advertise USB DFU support (i.e. as does the provided USB CDC ACM example).

The bootlaoder image will be flashed using SWD (Serial Wire Debug). For that you need an SWD adapter. The Makefile uses a ST-Link V2 along OpenOCD software. To flash the booltoader using SWD run rake flash_booloader.

Once the bootloader is flashed it is possible to flash the application over USB using the DFU protocol by running rake flash. To force the bootloader to start the DFU mode press the user button or short a pin, depending on the board. It is also possible to flash the application image using SWD by running rake flash_application.


SWD also allows to debug the code running on the micro-controller using GDB. To start the debugging session run rake debug.


The firmware offers serial communication over USART1 and USB (using the CDC ACM device class).