PiDMASound

Bare-metal audio playback on Raspberry Pi using only GPIO, DMA, PWM, and interrupts.

No operating system. No audio codec. No DAC chip.

Just hardware.

To play audio we need an analog signal. But, bare metal, the gpio can only output digital levels: 0 and 3.3V . How can we produce analog sound with bare-metal ?

Architecture

The engine converts PCM samples into a high-frequency PDM bitstream using a sigma-delta modulator. That bitstream is then streamed to the Raspberry Pi's PWM peripheral using DMA.

PiDMASound converts PCM audio into a 1-bit pulse density modulated (PDM) stream and outputs it through a single GPIO pin. A simple RC low-pass filter reconstructs the analog waveform, allowing the Raspberry Pi to play real audio with minimal external hardware.

Demo

PiDMASound can generate tones, sine waves, and simple melodies such as the Mario theme or Star Wars theme using only a single GPIO pin.

GPIO18 → RC filter → speaker

make mario
make starwars

Core Components

Sigma-Delta Modulation

PCM audio samples are converted into a 1-bit pulse density modulated stream using a first-order sigma-delta modulator.

Oversampling ratio: 64×

44.1 kHz × 64 = 2.8224 MHz PDM stream

The density of 1s and 0s represents the amplitude of the audio waveform.

When low-pass filtered, the original analog signal is reconstructed.

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PWM Serializer Output

The Raspberry Pi PWM peripheral is used in serializer mode.

Instead of generating duty cycles, the PWM hardware shifts out individual bits from its FIFO directly onto the GPIO pin.

Each bit becomes a high or low pulse, producing the PDM stream on GPIO 18.

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DMA Audio Streaming

The DMA engine continuously feeds data into the PWM FIFO using DREQ pacing.

Two buffers are used in a ping-pong configuration:

Buffer A → played by DMA
Buffer B → filled by CPU

Once DMA finishes a buffer, it switches to the other buffer automatically.

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Interrupt-Driven Refill

When a DMA buffer finishes, an interrupt fires at roughly:

~2756 interrupts per second

This is a memory optimization - it prevents from having to allocate one massive buffer, instead we allocate two small buffers that are alternatively pushed to the gpio, freed, refiled.

The interrupt handler:

1. Takes the next PCM samples
2. Runs sigma-delta modulation
3. Generates the next PDM block
4. Refills the completed DMA buffer

This allows continuous audio playback with minimal CPU usage.

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Hardware Setup

Required hardware:

• Raspberry Pi (BCM2835 compatible)
• speaker or headphones

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Repository Structure

asm/
  start.S
  interrupts-asm.S

src/
  audio-dma.c
  audio-dma.h
  sigma-delta.c

songs/
  mario.c
  starwars.c

asm/

Bare-metal code for interrupts and bootloader.

src/

Core audio engine including DMA configuration, PWM setup, and sigma-delta modulation.

songs/

Example melodies implemented using the audio engine.

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Realized as part of Stanford CS140E final Project.