MIDI→AUX Converter

Firmware for Arduino Nano (ATmega328P) that converts MIDI Note-On/Off messages into analog audio waveforms. Part of the АВК-6 «Музыкальная пауза» project.

Hardware

Pin	Function
PD0	MIDI input (RX via optocoupler, 31250 baud)
PD2	Optocoupler enable (active HIGH)
PD3	Audio PWM output (OC2B → RC filter, 160 kHz)
PC0	Potentiometer (ADC0, 0–5V)
PC1	Button (internal pull-up, mode switch + note off on press)
PD4	LED — Square wave
PD5	LED — Triangle wave
PD6	LED — Sawtooth wave
PD7	LED — Sine wave

Waveform Modes

Mode	Waveform	Potentiometer effect
0	Square	Duty cycle (0–100%)
1	Triangle	Rise/fall split point
2	Sawtooth	Ramp duration
3	Sine	None (fixed)

Short press the button to cycle modes. Note is silenced on press.

Building & Upload

pio run                 # build
pio run --target upload # build & upload

Remove MIDI jumper before upload — optocoupler gate (PD2) isolates RX.

Architecture

Fully interrupt-driven. Main loop does nothing but sleep (SLEEP_MODE_IDLE).

Subsystem	Timer	Interrupt	Priority	Description
Button	Timer1	TIMER1_COMPA_vect	High	10 ms tick, debounce 30 ms, calls noteOff() on press
Waveform	Timer0	TIMER0_COMPA_vect	Low	Generates PWM sample at note × 64 Hz
Audio PWM	Timer2	—	—	160 kHz Fast PWM on PD3, OCR2B updated by waveform ISR
ADC	—	ADC_vect	—	Free-running, updates potValue every 104 µs
MIDI	—	USART_RX_vect	—	Calls midiParser() directly, no buffers

Timer0 Prescaler

Period	Prescaler	OCR0A range
< 2040	8	62–254
≥ 2040	64	32–238

OCR0A clamped to ≥ 350 (44 µs period at prescaler 8) to prevent ISR cascade on high notes.

Memory

Resource	Used	Free
Flash	2586 bytes (8.4%)	28134 bytes
RAM	74 bytes (3.6%)	1974 bytes

Project Structure

platformio.ini
fw/src/
├── main.cpp          # init, sleep loop
├── uart.h / uart.cpp # MIDI UART (RX-only, ISR-driven)
├── midi.h / midi.cpp # Note period table, MIDI parser, noteOn/noteOff
└── waveform.h / waveform.cpp  # Sine table, waveform ISR, 4 modes

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Project created with KiloCode agent, model deepseek/deepseek-v4-pro.