Starting about 74 as tech on anything DC to 1Ghz. Most of computers were 16 bits or more. Some were the micro tube, many small vacuum tubes mounted to board, Some were transistor based. Can not think of a logic type I have not worked on back then. One system used TTL & ECL with pc boards bonded to copper plates. Those first flat packs were fun.
To give you idea of site size, Around that time had a circuit breaker fail, 660V 400A & problems with 100Kw no break power( think UPS with out limit).
First Z80 I helped design and build, the Z80 chip cost $250 at the time.
Did some Nova1200 programming. When I really got in to DEC PDP-11's, RCA let me use their $200,000 PDP-11 development system when not in use.
I bought my first microprocessor 'development system' (HUG1802, similar to the COSMAC ELF) in 1980. It cost NZ$300 for the kit - a lot of money for a trainee technician whose annual salary was only $3700! I used this to learn about microcontrollers, machine code programming etc.
Around that time i worked in research. They had full 1802 development system with dual 8" drives. Know a fab change to 1802 that added protection on pins stopped it working at high temp. Old fab version worked up in 150C+. Not my work thing.
My thing was help in design of 8085, z80 & 68000 systems & programming,
Also High temp testing. Tests where wrong solder was a fail.
I forget what temp we had a 8085 running at.
Had access to development systems of 8080, 8085 & Z80
About the time the PCxt demo came I had a 68000 on one work desk. At times would load in $5000 of memory to get some compute work done,
Have had a home computer from about 76. My last Z80 system had 6 disk drives. Biggest Z80 system I worked on had 18 Z80's in box connected to other boxes.
Not much need at home when I could stay late at work an play.
If you need to see a good/not bad Z80 design, you should look a TRS80 model 12. Really shows how to connect many SIO, PIO & DMA together. The whole system becomes an IO processor for the 68000 in the Model 6000.
So Short answer, when not fixing/maintaining something, had time and access to a lot of INFO. Schematics, theory of operation and data sheets.
If it stopped I had to fix it.
Best processor board I helped design was a 68k that could connect to two system buses with local expansion, Many on each bus.
the idea of using AVR8 is smart but not clever (especially in the educational's context) for a simple reason: it's slower than the CPU (especially if programmed with Arduino in mind), which requires people to strictly understand timing between the Z80's bus and AVR8's. Not so simple to understand, especially for beginners.
I agree. It could be a good advanced project for someone who already understands the basics.
Could be that you are looking at it the wrong way.
Z80 goes full bore until it receives a WAIT Input, Then stops DEAD. You have a static system at this point and all the time in the world unless you have something like dynamic ram in system. So at this point any speed is fast enough. The only critical spot is holding data on bus for a read and then getting off bus before Z80 uses it after read. And even this is just a case of controlling the Z80 Clock.
Here it is even easer, You are user not a developer, you are just connect wires and use.
So how hard is the timing to understand as a user. When Z80 Wait goes low you are in a memory or IO cycle. Address is valid, MREQ or IORQ valid. If Write then write data valid. If Read then waiting on read data to be valid.
So the only critical spot in timing is getting WAIT asserted fast enough.
So what if you have a slow working system to start with. The only catch here is the quality of software on the ATMEGA. And even here you have examples to follow on how to add.
It may not be what many want as a final system, but fine as long as there is no time critical data to process.
So I see you both backwards in your thinking, It's great for beginners. A lot easer to wire up then any of the examples you might create for a beginner.
With so few wires a lot less problems with a wire error. And if polished software existed, the software could assist in checking the wires.