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z80-homebrew

Z80 Home Brew Micro-computer Project

Having watched so many retro computing videos on YouTube I thought I would relive some of my youth by building something simple. I remember the old electronic magazines of the 1980's which had plenty of simple programmable computing devices to make at home. Always wanted one. Now I will do it - only a few decades behind :-)

6502 vs Z80

My first home computer was a Vic20. Not long after having that in my grubby mitts I was writing 6502 machine code, hand assembling as we all did back then. Later I moved onto an Amstrad CPC464 and dived into Z80 which I then took to the Amstrad PCW 512.

Looking around at parts I found it hard to locate any good 6502 parts and with the vast Z80 community and lots of great material to dust off my rusty electronics skills I chose that.

Therefore this project, is Z80 based. Turns out that is a good choice as the arch is very simple with a load of great chips that tie closely together. Makes for an easier life.

Update: That said, with the PDIP Z80 now out of production so so great... But not completely lost :-)

Current Specification/Feature Set

  • Z80 4/8/10MHz
  • 32K ROM
  • 32K RAM
  • PIO 1: Port A & B - Controlling LCD and keyboard:
    • Mini: 20x4 LCD and 4x4 matrix keypad
    • Mega: 20x4 or 40x4 LCD and 50 key matrix keyboard
  • PIO 2: Port A - Persistent storage via Microchip serial EEPROMs.
    • Max 320K on board (5 x 64k). Currently set for 32k x 5.
    • Uses own file system with max file size of 16k
    • 5 banks of independant storage each with own bank label
  • PIO 2: Port B - Exposed 8 bit bus and SPI bus. Used for more storage (512k/1024k), SPI devices or digital I/o
  • Auto startup scripts/demos built into ROM and/or created on storage.
  • Exposed RC2014 connector
  • Built-in address decoder for four devices (2 used above)
  • Tape support in progress on Device A by default and can be plugged into the bus. Long term on board.
  • On board sound support (TODO on device B)
  • Sound chip via SPI
  • External cart support via PIO 2 Port B. Adding support for:
    • Real time clock SPI (TODO)
    • Extra storage via SD and/or Microchip serial EEPROMS (TODO)
    • RP Pico using SPI for extra features such as wifi network support via PicoSPINet addon
    • GPIO on the 8 pins (TODO) and independant control use of SPI bus
  • Feature to load a user dictionary from file storage at boot time
  • Highlevel and low level storage words
  • Hardware abstraction layers to allow for common entry points and drivers for different keyboards, screens, etc
  • Multiple selectable frame buffer support (currently three)
  • Built-in Forth OS (inspired by Jupiter Ace). Language defintion and progress etc in FORTH.md
  • Built-in full monitor for break points at all key points.
  • Using my own very simple malloc/free routinues or can switch to DK88 project version with defines
  • 3d printed case designs in 'case' directory.
  • To aid in board development and speed up the change and test cycle I've now added support for running on the wonderful Steven Cousin's SC114 board. This is quicker than having to remove the EEPROM (even in a zif socket), burn it and put it back.
  • Added CP/M support for even quicker test cyles via RunCPM!

Expansions/Addons

  • PicoSPINet - A Pico device which provides 5/6 SPI network ports allowing intercoms between nodes and act as an internet gateway.

    • Provides NTP services
    • Internode communications
    • Internet communications
    • Storage services
    • Powered nodes
    • Emulates the EEPROM used for onboard storage with node specific and common storage banks

    For more details look at the Addons README

  • Tape support on Device A (WIP)

  • SPISound - Using a shift reg and the classic SN76489 sound chip we have four channel sound via SPI.

    For more details look at the Addons README

  • Tape support - Currently in progress

Progress

Check the DEV-DIARY.md for progress and what I learnt - spoiler, it is a lot!

Stage 1.x was very very basic using a Microchip PIC to bitbash code into RAM or manually as I didn't have an EEPROM programmer.

Stage 2.x got us to strip board in varying degrees of success

Stage 3.x with a propper PCB things are much more stable (and faster clock freq)...

Stage 4.x a useable machine now with many expandable features.

Current Forth word list in WORD-LIST.md along with how I have put together my version of FORTH in FORTH.md.

Stress testing FORTH OS

[https://youtu.be/hWfx7d5nqZw]

A simple game of 'Simon Says'

: sgen #1 do #1 #5 rnd dup #48 + i scratch ! .> bl .> loop ;
: isay #1 #0 scratch ! ; 
: nsay #0 scratch dup 1+! @ ;
: tsay #4 pause cls Your-go .- #10 #1 at accept ;
: schk #1 scratch #0 #0 scratch @ substr swap drop compare ;
: bsay "Your best was" #0 scratch @ #1 - num2str info ;
: ssay isay repeat cls nsay sgen tsay schk #1 = until bsay ;

[https://www.youtube.com/watch?v=0pCNQlxzYs0]

Parts

At each stage I will document and attach the schematics for mostly for my own reference should things go wrong.

References

I found so much great advice and inspiration around the net here are some of those that made the most impact (in no particular order):

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Z80 Home Brew Micro-computer Project

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