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Electronics -> The Z80 project

Last modified on Wed, 16th Apr 2008 at 14:41 BST by zipplet
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Latest updates


Wednesday, 16th April 2008 - I've been very busy with work and life hence the lack of progress. Anyway, I've found out that I can use the Parallel Slave Port in the 16F877A to easily interface the supervisor to the Z80 I/O bus. This greatly simplifies design, improves speed and frees up 8255A pins.
Monday, 7th April 2008 - Swapping chips to test a single opcode change in the BIOS is beginning to get on my nerves so I am looking into making a simple EPROM emulator I can leave plugged into the Z80 board. This way I can upload and test a code change pretty much instantly, without almost snapping a pin on a £10 part! Update: I've decided to instead for now build a small bus interface card that will plug into one of the expansion sockets and allow a PC to read and write to the address bus on demand. I've tidied up the BIOS code and gotten it to partially complete POST - tomorrows task is to start work on the supervisor PIC.
Saturday, 5th April 2008 - I've been busy but tonight I am going to get that second SRAM socket working. Also I found a nicer assembler which I am now using.
Wednesday, 2nd April 2008 - Motherboard working! I've soldered down enough for the LCD display to work again. I need to solder up the second RAM socket and then look at some way of adding input.
Monday, 31st March 2008 - I've laid out the motherboard, soldered down the connectors and IC sockets, and prepared it for populating. Soon I should have a working motherboard for this system with expansion card sockets! Also, today my Z80 CTCs arrived, another crucial component of my system which will generate interrupts on a timer for multitasking. The current motherboard design has 2 expansion card ports, adding more is impractical right now. I've left an empty space so I can incorporate the supervisor PIC + Z80 CTC directly onto the motherboard later.
Saturday, 29th March 2008 - On the picture above you can see a nice 20x4 HD77480 LCD being driven by the hardware. Nice isn't it?
Friday, 28th March 2008 late night update - I got the 8255A PIO chip working successfully!
Friday, 28th March 2008 - Today I got a basic system working on breadboard, with a simple 8-bit parallel port connected to LEDs. Video -> http://uk.youtube.com/watch?v=wntSq63LX48 - in this video I give a brief overview of the core of the system.

Documentation


I aim to eventually provide enough documentation for someone else to be able to build a compatible clone of my system for fun or to learn how such a system works. Later on I'll have fancy circuit schematics and firmware downloads.

What is it?


The Z80 project is something I've wanted to do for a long time - build a complete microcomputer system from parts.

At the moment the specs will be:

Addon cards I intend to make:

I've ordered some variable frequency oscillator modules so I can use a rotary switch to set a range of CPU clock speeds. There will be a row of LEDs used to show bus status on the front panel, like those old Altair computers.

The Zipplet80 OS


A small OS called Zipplet80 will run on this machine. It will provide very primitive semi-preemptive multitasking. Multiple programs may be loaded into memory at once and interrupts will be used to switch between them.

A few things I need to implement:

More pictures in roughly chronical order


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This is the first breadboard version you can see in my first youtube video.

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LCD display working! Still on breadboard.

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Initial prototyping of the first motherboard layout.

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Yay, it lives! At this point I still need to wire up the second SRAM socket. This is a lot more stable than the breadboard version and of a much better design with pullup resistors in the right places and more capacitors added for filtering.