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Computer on a Chip: FOSS silicon clone of Atari 2600

Goal of this project is to create a complete computer on a single chip using only Free and Open Source (FOSS) tools and Open Source PDK.

Atari 2600 fits this goal perfectly thanks to it is minimal hardware design (i.e. only 128 bytes of RAM) while sporting a large software library!

ASIC Fabrication

The design is so small that around 40 Atari consoles fit on a silicon die! The silicon area of this design is 0.2 mm2 (1000um x 216um) on a SkyWater 130 nm node.

Explore chip in 3D directly in the browser!

This design is submitted for fabrication with Tiny Tapeout 09 via eFabless CI 2411 Shuttle. Check status here: https://app.tinytapeout.com/projects/1343

Below are integrated circuit layout and metal2/metal3 layer connectivity between the logic gates. The layout was made with automatic place-and-route flow of OpenROAD using open-source 130 nm SkyWater PDK.

Implementation overview

This design is based on Lawrie Griffiths' FPGA implementation of the Atari 2600 written in Verilog with the following modifications:

  • Arlet Ottens' 6502 version of the CPU core
  • a single clock domain for all components
  • scanline doubler to convert NTSC 15 kHz to 30 kHz VGA video signal on the fly without introducing a framebuffer
  • dithering to approximate 128 colors over the 6 bit-per-pixel TinyVGA PMOD
  • QSPI to access cartridge ROMs stored on external Flash
  • fixes to TIA and PIA (I/O & Timer part of the RIOT) chips
  • only NTSC is supported

Modifications to Arlet's 6502 core:

  • reset cycle bug fixes such as writes enabled while address bus is still in undefined state
  • reset cycle explicit register initialisation because unlike FPGAs real silicon can not just set default values

Brief overview of the Atari 2600

The Atari 2600, also known as the Atari VCS, is a second-generation home video game console released in September 1977. The system is built around three chips:

  1. Main CPU: MOS Technology 6502 microprocessor, clocked at 1.193182 MHz.
  2. Video & Audio Processor (TIA): The Television Interface Adapter (TIA) clocked three times faster than the CPU at 3.579545 MHz. It generates NTSC video output and provides two audio channels.
  3. RAM, I/O, and Timer (RIOT): MOS Technology 6532, also known as the RIOT chip, integrates 128 bytes of RAM, input/output capabilities, and a timer within a single package.

Of these, only the TIA was custom-designed for the Atari 2600. The 6502 CPU and the RIOT chip were standard off-the-shelf components manufactured by MOS Technology.

Unique aspects that make Atari 2600 suitable to fit on a tiny piece of silicon:

  1. Console had no framebuffer. Graphics are generated by "racing the beam" technique.
  2. Only 128 bytes of RAM.
  3. All games code and data are completely external to the system and are situated on a pluggable ROM cartridge. Roughly 1 MB/s of memory bandwidth.
  4. Relatively low design complexity totaling in approximately 15K tranistors: 4528 (CPU), 6193 (TIA), ~5000 (RIOT).
  5. Original designs are well studied and schematics are known down to a single transistor.

Hardware Schematics

  • PCB / system level schematics of Atari 2600.
  • 6502 reverse engineered circuit.
  • Original TIA schematics.
  • Reverse engineered RRIOT 6530 closely related to the 6532 RIOT in the console.

Future plans

How to build & run

Verilator

FPGA

Cocotb testing environment

Games and demos tested

PMODs

How it works?

What is Tiny Tapeout?

Tiny Tapeout is an educational project that aims to make it easier and cheaper than ever to get your digital and analog designs manufactured on a real chip.

To learn more and get started, visit https://tinytapeout.com.

Resources