Polyphonic Keyboard Synthesizer

Embedded Rust on STM32

Project Documentation & Implementation Plan

info

Author: Delia Udrea
GitHub Project Link: https://github.com/UPB-PMRust-Students/project-dm-2025-DeliaUdrea

Description

This project aims to develop a fully functional digital musical instrument capable of rendering polyphony (chords) and responding in real-time to user interactions (keys, sustain pedal, pitch bend).

The core of the system is an STM32 Nucleo microcontroller, programmed in the Rust language (using a Real-Time framework) to guarantee minimal audio latency and high reliability.

Functionality

1. Polyphonic Input: Reading approximately 61 keys (organized in an $R \times C$ matrix) with anti-ghosting protection (diode per switch).
2. Audio Synthesis: Digital generation of sound waves (sine/square) on the microcontroller, utilizing the DAC or PWM peripheral.
3. Expressive Control: Implementation of the Sustain function (pedal) and Pitch Bend control (joystick).
4. Audio Output: Amplifying the weak digital signal from the MCU to drive an external speaker.

Motivation

2.1 Project Motivation

• Technical Challenge: The project combines electronics (key matrix, audio filtering) with real-time programming, serving as an excellent demonstration of Rust capabilities in the no_std (embedded systems) environment.
• Experience Quality: The choice of high-quality mechanical switches (Outemu Linear, 65g) over simple buttons offers a superior feel suitable for a musical instrument.
• Practical Application: Creating a functional digital instrument capable of playing complete songs.

2.2 SWOT Analysis

Strengths	Weaknesses
High Performance: Rust no_std ensures minimal latency for real-time audio.	Complexity: Manually wiring a 61-key matrix is labor-intensive and error-prone.
Durability: Mechanical switches provide a robust and long-lasting interface.	Audio Quality: PWM synthesis is simpler but lower fidelity than a dedicated DAC chip.
Polyphony: Capable of playing multiple notes simultaneously (chords).

Opportunities	Threats
Scalability: The design can be expanded to include more effects (reverb, echo).	Component Availability: Specific switches or STM32 boards may be out of stock.
Educational: Deep dive into embedded systems, digital signal processing, and Rust.	Timing constraints: Real-time audio requires strict timing; missed deadlines cause glitches.

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Architecture

2.1 Hardware Architecture

The system is divided into three main interconnected blocks:

1. Input Block (Keyboard): The $R \times C$ matrix of switches and diodes.
2. Control and Logic Block (STM32): Processing input and generating the digital audio signal.
3. Output Block (Audio): Filtering, amplification, and playback of the sound via the speaker.

Logical Interconnection Diagram:

2.2 Software Architecture

The software is built using Rust and the RTIC (Real-Time Interrupt-driven Concurrency) framework to manage the strict timing requirements of audio synthesis. The architecture binds strictly to the hardware peripherals to ensure zero-latency performance.

Concurrency & Priority Model

• Audio Task (Priority 2 - Critical): This is the "heartbeat" of the synthesizer. It is triggered by a hardware timer (e.g., TIM2) at exactly 44.1 kHz. Its sole responsibility is to calculate the next sample value and write it to the DAC/PWM output. It preempts any lower priority tasks to ensure no audio glitches occur.
• Logic Task (Priority 1): Triggered by software (Software Interrupt) whenever a key state changes. It handles the musical logic: converting key indices to MIDI notes, calculating frequencies using lookup tables (LUT), and processing Envelope (ADSR) states.
• I/O Task (Priority 0): A periodic scheduled task (approx. every 5ms). It handles the physical interface (Matrix Scanning and ADC polling).

Hardware Binding

The software interacts directly with the STM32F4 peripherals via the stm32f4xx-hal crate:

1. GPIO (Key Matrix): The Key Scan algorithm dynamically reconfigures GPIO pins. It drives one Row pin High (Logic 1) and reads the Column pins. If a Column reads High, the switch at that intersection is closed. It iterates through all rows rapidly to detect multiple key presses.
2. DAC / PWM (Audio Output): The system uses a hardware Timer to enforce the sample rate.
   • PWM Mode: The timer controls the duty cycle of a high-frequency square wave (e.g., >200 kHz) to approximate an analog voltage through the RC filter.
   • DAC Mode: The calculated digital sample (12-bit) is written directly to the DAC Data Holding Register (DHR), which converts it to voltage.
3. ADC (Expressive Control): The Joystick and Pedal are connected to Analog Inputs. The code triggers an ADC conversion sequence and reads the raw 12-bit values (0-4095) to calculate Pitch Bend modulation and Sustain status.

Software Functional Diagram:

Log

This log tracks the project's progress, from component acquisition to the implementation of key functionalities.

Week 1: Planning and Hardware Acquisition

Date	Status	Observations
Day 1-2	Finalized BOM	Final confirmation of components (Outemu, PAM8403, 1N4148).
Day 3-5	Workspace Setup	Installation of Rust toolchain (cargo-embed, probe-rs).
Day 6-7	Hardware Design	Creation of logic schematic and interconnection diagram.
Status:	90% Complete	Component acquisition is the priority.

Week 2: Basic Tests and Soldering

Date	Status	Observations
Day 8-10	Matrix Soldering (Ph.1)	Soldering first 20 diodes. Wiring 4x4 test matrix.
Day 11-12	GPIO/Input Test	Rust code to scan 4x4 matrix and verify anti-ghosting.
Day 13-14	Audio Output Test	Generating fixed frequency tone using DAC/PWM.
Status:	30% Complete	Confirmation of basic input/output functionality.

Week 3: Real-Time Implementation and Polyphony

Date	Status	Observations
Day 15-17	Audio Task	Implementing Audio Task at max priority (constant sample rate).
Day 18-20	Polyphonic Synthesis	Modifying Audio Task for simultaneous notes/chords.
Day 21	Final Assembly	Finalizing soldering of all ~61 keys.
Status:	70% Complete	Basic piano functionality is finalized.

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Hardware

3.1 Critical Components

Component	Specifications	Role in the Circuit
Microcontroller	STM32 Nucleo (e.g., F401RE)	Processing, Timers, ADC, DAC/PWM.
Switches	Outemu Linear (3-pin), 65g	Ensures quality tactile input.
Polyphony Diode	1N4148	Prevents "ghosting" in the $R \times C$ matrix.
Amplifier	PAM8403 Module ($2 \times 3$ W)	Audio amplification from 3.3V to speaker level.
Matrix Wiring	Set 5 Spools 24 AWG	Stable, soldered wiring for the ~70 internal connections.

3.2 Logical Interconnection Diagram

The hardware blocks connect to the STM32 peripherals as follows:

• STM32 Box: Handles GPIO, DAC/PWM, and ADC pins.
• Matrix: $R$ rows connect to GPIO (Output) and $C$ columns connect to GPIO (Input with Pull-up).
• Audio: Connection runs from the DAC/PWM pin, through the RC filter to the input of the PAM8403 module.

3.3 Detailed Electronic Schematic (KiCad)

• Key Matrix: $R \times C$ connection with each diode (1N4148) correctly oriented on each switch.
• Audio Filter: Resistor and capacitor (RC Low-Pass Filter) needed to smooth the PWM signal.

3.4 Prototyping and Wiring

• Matrix Construction: Will be built by manually soldering the 24 AWG cable to the output pin of each switch.
• Termination: The approx 20 wires from the matrix will be soldered onto the 20-pin IDC Connector.

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Bill of Materials

Estimates based on average market rates (RON).

No.	Component	Specifications	Qty	Unit (RON)	Total
HARDWARE
1.	Microcontroller	STM32 Nucleo (F401RE)	1	80.00	80.00
2.	Key Switches	Outemu Linear, 65g	70	1.80	126.00
3.	Diode Kit	1N4148 (20 pcs/kit)	4	12.00	48.00
4.	Audio Amplifier	PAM8403 (2 x 3W)	1	25.00	25.00
5.	Speaker	4 inch, 8 Ohm	1	40.00	40.00
6.	Sustain Button	Momentary Pushbutton	1	10.00	10.00
PROTOTYPING
7.	Protoboard	Breadboard MB102	1	20.00	20.00
8.	Jumper Wires	Male-Male	1 set	15.00	15.00
9.	Matrix Wire	24 AWG Cable	1 set	30.00	30.00
10.	Matrix Conn.	IDC 20 pin	1	10.00	10.00
11.	Speaker Cable	2 x 0.75 mm^2	1 m	5.00	5.00
TOOLS
12.	Soldering Kit	60W Iron + Stand	1 set	125.00	125.00
13.	Resistor Kit	600 pcs	1 set	45.00	45.00
14.	Capacitor Kit	500 pcs	1 set	50.00	50.00
		TOTAL ESTIMATED			~ 619

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Software

5.1 Detailed Software Architecture

The system relies on a concurrent architecture, likely using RTIC or Embassy to ensure real-time responsiveness.

Task	Function	Priority	Description
Audio Gen	Audio Output	Max (Critical)	Runs at ~44 kHz, feeding DAC/PWM. Must not be interrupted.
Key Scan	Matrix Input	Medium	Scans matrix (1-5 ms) to detect key presses.
Control	Musical Logic	Medium	Calculates Hz and Volume, manages Sustain/Pitch Bend.
Idle	Debugging	Low	Sends debugging messages to PC (RTT).

5.2 Shared Data Structure (Safety in Rust)

To prevent data races, all data accessed by both the Audio Task and the I/O Task is protected.

• Active Notes: A vector or array (e.g., Vec<NoteData>) storing currently active notes.
• Protection: Protected by critical sections (RTIC) or channels (Embassy).

5.3 Functional Diagram (Data Flow)

1. Physical Input: Switches, Joystick, Pedal.
2. Scan Task: Matrix is scanned $\rightarrow$ ADC pin is read.
3. Key Event: NoteOn / NoteOff sent to Musical Logic.
4. Control Logic: Determines Freq/Amp, updates shared structure.
5. Audio Gen Task: Reads shared structure, calculates wave value.
6. Output: Digital signal $\rightarrow$ Filter $\rightarrow$ Amp $\rightarrow$ Speaker.

Links

1. Embedded-Graphics Documentation
2. STM32 Nucleo Datasheet
3. Rust Embedded Book
4. RTIC Documentation