Raspberry Pi 1 Bare-Metal Synthesizer

This project turns an original Raspberry Pi (Model 1 B/B+, Zero, or Zero W) into an instant-on, single-voice synthesizer that runs without any operating system (bare metal).

It boots in under two seconds and produces sound and graphics with minimal external components.

Features

• Instant-On: Boots directly into the synth application in ~1-2 seconds.
• Audio Output: Generates a sawtooth waveform on the 3.5mm composite audio jack.
• Video Output: Displays a simple amplitude visualizer on the composite video output.
• Looped Melody: Plays a pre-programmed random melody, no input required.

Requirements

Hardware

• Raspberry Pi: Model 1 (A, B, B+), Pi Zero, or Pi Zero W. This code is for the BCM2835 SoC and will not work on Pi 2, 3, 4, or 5.
• SD Card: A microSD or standard SD card (depending on your Pi model), 1GB is more than enough.
• Power Supply: A standard 5V micro-USB power supply.
• Audio Cable: A 3.5mm audio cable to connect to headphones or a speaker.

Software

• ARM Cross-Compiler: The arm-none-eabi-gcc toolchain is required to build the kernel.
• Raspberry Pi Firmware: You need the bootcode.bin and start.elf files from the official Raspberry Pi firmware repository.

Quick Start with Scripts

For a fast and easy setup, you can use the provided automation scripts.

1. Build the kernel:

   ./build.sh
   

   This will compile the C and Assembly code and create the kernel.img file.

2. Deploy to SD Card: Insert your FAT32-formatted SD card. Find its device name (e.g., /dev/sdX or /dev/mmcblkX) and mount point.

   # Example: ./deploy.sh /media/user/RASPIBOOT
   ./deploy.sh <path_to_sd_card_mount_point>
   

   This script copies kernel.img, bootcode.bin, and start.elf to the SD card and then safely unmounts it.

Manual Setup and Deployment

1. Install Toolchain

On Debian/Ubuntu-based systems, you can install the cross-compiler with:

sudo apt-get update
sudo apt-get install gcc-arm-none-eabi binutils-arm-none-eabi

2. Get Raspberry Pi Firmware

The Pi's GPU first loads firmware from the SD card before handing control to the ARM core. You need two files:

• bootcode.bin: The second-stage bootloader.
• start.elf: The GPU firmware.

You can download them from the official Raspberry Pi firmware repository. Make sure to grab the files from the boot directory. For this project, you only need these two files.

Place them in the root of this project directory so the deploy.sh script can find them.

3. Build the Kernel

With the toolchain installed, compile the synthesizer by running make:

make

This command uses the Makefile to compile boot.S and kernel.c, linking them into a final binary image named kernel.img.

4. Prepare the SD Card

Your SD card must be formatted with a FAT32 filesystem. Most new SD cards are already formatted this way. The partition should also have the "boot" or "lba" flag set, which is standard for bootable Raspberry Pi cards.

5. Deploy to SD Card

Mount the SD card and copy the three essential files to its root directory:

• bootcode.bin (from the firmware repo)
• start.elf (from the firmware repo)
• kernel.img (generated by the make command)

After copying, unmount the SD card safely.

Hardware Connections

1. SD Card: Insert the prepared SD card into your Raspberry Pi.
2. Audio: Connect headphones or a speaker to the 3.5mm audio jack.
3. Video (Optional): Connect a composite video cable from the RCA jack (on Model B) or the 3.5mm jack (on later models) to a compatible display.
4. Power: Connect the 5V micro-USB power supply. The Pi will boot immediately.

Usage

Once powered on, the synthesizer will automatically start playing a random melody loop. The composite video output will show a simple red bar that visualizes the audio amplitude.

There is no user input required for this version of the code.

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This project is for educational purposes and demonstrates low-level hardware control on the BCM2835 SoC.