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Homebrew 8-bit TTL CPU
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HOL-1 a homebrew 8-bit TTL CPU

HOL-1 is a TTL based 8-bit CPU based on the 'simple as possible' breadboard design by Ben Eater

As well as adding my own modifications to each module of this CPU. i.e.

  • more memory
  • more registers
  • more ALU functions
  • more commands (assembler code to follow)

The main change is that HOL-1 is designed for permanent construction using common 8cm x 12cm double-sided perfboards.

For more on HOL-1 see

Design Principles

  • A working CPU is more important than performance or perfection (I'm learning as I go!)
  • Using basic TTL 74LS family IC's from circa mid-70's. i.e. an ALU on one chip would be easier but where's the learning in that
  • 8cm x 12cm double-sided perfboards = a 30 x 42 grid of 2.54mm spaced vias
  • Initial design is on Fritzing (better suited to perfboard layouts but not suitable for Git)
  • KiCad is then used for a more professional(ish) schematic and PCB design (and a more suitable format for Git)
  • The KiCad PCB will adhere to the perfboard grid layout for component placement but tracks will then use a finer grid. This is because I'm not making PCBs but mass wiring the perfboard in wirewrapping style.
  • Blinkenlights - plug-in status LEDs at every opportunity that could be swapped for control panel fly leads in the future
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