Maintaining and building on the code from http://wiki.openmusiclabs.com/wiki/Digix0x, as found at https://www.modwiggler.com/forum/viewtopic.php?t=289244

digix0x TB-303 Emulator

A digital emulation of the Roland TB-303 synthesizer, running on SAMD51-based microcontrollers (e.g., Adafruit Metro M4/ItsyBitsy M4). This project recreates the iconic 303 sound with real-time audio synthesis, envelope shaping, accent and slide features, and a step sequencer.

Features

• Classic TB-303 style sawtooth oscillator
• Four-pole resonant diode ladder filter emulation
• Accent and slide (portamento) handling
• Step sequencer with rest, accent, and slide flags
• Real-time synthesis in a timer interrupt for authentic analog behavior

Hardware Requirements

• Microcontroller: SAMD51-based Arduino-compatible board (e.g. Adafruit Metro M4 Express, ItsyBitsy M4)
• Controls: Potentiometers or CV sources for:
  • Resonance (A2)
  • Cutoff (A3)
  • Envelope Modulation (A5)
  • Decay (A4)
  • Accent (A1)
• Audio Output: Use A0 (onboard DAC if available) or an external DAC (see below)
• Power: 3.3V or 5V (check your board’s requirements)

Audio Output Options

1. Onboard DAC (Recommended for Simplicity)

• Many SAMD51 boards provide a 12-bit DAC on A0.
• Use analogWrite(A0, value); in the code.
• Add a simple RC low-pass filter (e.g., 1kΩ resistor in series with output, 10nF capacitor from output to GND) for audio smoothing.

2. External DAC (For Higher Quality)

• MCP4725 (I2C) or MCP4921 (SPI)
• Wire VCC, GND, SDA/SCL (I2C) or MOSI/SCK/CS (SPI) as per your DAC and board.
• Use the appropriate Arduino library (e.g., Adafruit_MCP4725).
• Replace all analogWrite(A0, ...) calls with your DAC’s output function.
• Note: ItsyBitsy M4 doesn't support i2s DACs like PCM5102 source

Software Requirements

• Arduino IDE (or PlatformIO)
• Board Support Package: Install “Adafruit SAMD Boards” via the Arduino Boards Manager.
• Libraries:
  • SAMD51_InterruptTimer.h (or a compatible timer interrupt library, e.g., Adafruit_ZeroTimer)
  • For external DAC: corresponding libraries (see above)
• Required Files: Ensure these header files (lookup tables) are in the same directory as 303_m4_fusion.ino:
  • 303filt_tanh.h
  • sinetable.h
  • midi_note_table_50x64.h
  • 303filt_coeff_5us.h
  • 303_vcf_decay_table.h

Setup and Usage

1. Connect Controls:
   Wire potentiometers (or CV) to the analog pins as listed in Hardware Requirements.
2. Audio Out:
   Connect A0 (with a low-pass filter) to an amplifier or audio interface.
3. Install Libraries:
   Install any required libraries via Arduino Library Manager or PlatformIO.
4. Open and Compile:
   Open 303_m4_fusion.ino in Arduino IDE.
5. Upload:
   Select your board and port, compile, and upload.
6. Play:
   The built-in sequencer will play a preset pattern; adjust the pots for classic 303 tweaking.

How It Works

• All sound synthesis and sequencing is performed in a high-speed timer interrupt (myISR()).
• The code emulates the classic TB-303’s sawtooth oscillator, diode ladder filter, envelope generators, accent, and slide.
• Patterns, slides, and accents are hardcoded in the sequence[] array, but can be edited for new patterns.

Customization

• Patterns:
  Edit the sequence[] array for your own sequences (see comments in code).
• Filter/Envelope Behavior:
  Adjust coefficients and tables for different response curves or time constants.
• External Control:
  Modify loop() to add MIDI or CV/Gate input for more flexible sequencing.

References

• Roland TB-303 background
• Adafruit Metro M4 Express
• Adafruit MCP4725 DAC