A synthesiser written for the Cortex M4F based STM32F3Discovery, which has the STM32F303VC chip.
I decided against using any libraries or prewritten HALs as I wanted to explore the different ways of abstracting hardware access in C++, and gain a deeper understanding of the underlying hardware.
Uses a sine wave sample with a hardcoded envelope to generate sound. Accepts MIDI note-on and note-off signals.
Requires an STM32F3discovery board, a MIDI breakout board, and an audio jack.
The MIDI input should be connected to GPIO pin B7, and audio output should be connected to GPIO pin A4.
Download and compile using CMake:
cd greesy/
mkdir build && cd build
cmake ..
make
You can then connect the board to your computer and flash it using
st-flash write main.bin 0x08000000
Connect a MIDI keyboard to your MIDI input and play!
A sine wave sample and an amplitude envelope is hardcoded in an array in tables.hpp.
Using a floating point index and linear interpolation, these are indexed into roughly every 22μs (by a timer interrupt).
The result from the amplitude envelope is then applied to the result from the sine wave table and is sent to the DAC (Digital-Analogue Converter) peripheral which outputs it as audio.
The MIDI input is connected to the USART1 peripheral of the board. Whenever data is received to this peripheral, an interrupt is used to write it to a (large) circular buffer. The main loop then checks for any pending MIDI instructions in this circular buffer, and if any are present sends them to the synthesiser module.