I’ve been researching and playing around with schematics for a few days now and I think I’m getting somewhere. A non-exhaustive list of what I have is:

• Z80 CPU
• Z80 PIO (Parallel IO)
• 8MHz oscillator
• 8Kb EEPROM
• 32Kb SRAM
• ATmega 8515 for external programming (more on that later)
• Switches
• LEDs
• 40 pin exapansion header
• Altera CPLD for address decode

External Programming

One of the things I’d like to have in this design is an easy way to program the EEPROM within the circuit to create something more self contained. Currently I have an ATmega 8515 in the design - mainly because it’s the cheapest AVR on RS with enough pins to do this. It sits on the address and data busses so it should be possible to have it read and write the SRAM as well. Unfortunately it can’t use the IO components because there weren’t enough pins on the microcontroller for the IO toggle line. I have an idea though that might allow it access to more pins - or maybe even allow using a smaller microcontroller. The only issue is it requires some extra ICs and the part count is getting quite high!

Next Steps

In no particular order, the next steps are:

• Breadboard out some of the ideas to check them
• Start designing a PCB from the schematic
• Write some z80 examples
• Write an easy to follow tutorial 
• Design the address decoder
• Write the AVR code to implement the programmer

Other thoughts

Right now there are a few things that need more thought:

• Would it be better to use discrete logic instead of the CPLD?
• Should the microcontroller use bidirectional shift registers as a port expander for the data and address lines?
• is this actually going to work? ;-)