A cheapo Altair 8800 style computer using a Z80 - (part 4) - Run/stop function and programming

In this series of videos we make a Z80 computer in the style of an Altair 8800 for people/budgies who can't afford £4k+ for a box with a few LEDs in it.
In this video we make the run/stop module on a breadboard and run a test program.
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If you want to build along at home you'll need:
1 breadboard
1 74HC74
2 push button
2 1k ohm resistor
a selection of breadboard wire

[@herberttlbd]

I was prepared to get started building my own Altair 8800 bui, for some reason, I've now got the urge to watch all of Bottom again.

[@DrBudgieandtheHumanMan]

I can't blame you really, Bottom is the best! You could watch it and build at the same time! (that's how I fail exams).

[@colonelbarker]

Really enjoying this series. I'm thinking about following along- But I'm curious to see how far you're going to take it along. Are you going to make a fancy front panel? 😃

[@DrBudgieandtheHumanMan]

Thanks mate, glad to hear you're enjoying it! The plan is to make a fancy front panel, we've got an old bit of lab equipment for the enclosure and we'll probably use some mild steel (as terrifying as it sounds, I'm letting the human man play with power tools) for the front panel and try and paint it up like a real life 70s computer! We've also got some fancy PCBs on the way too! Whether you wait till the end or get started now, I hope you do give it a go it's a fun build! :)

[@colonelbarker]

@@DrBudgieandtheHumanMan wooo. Fancy pcbs. That sounds amazing. The human man better watch out playing with power tools. I've been caught out a few times with z80 and 6502 builds on KZbin where people give up making them three or four videos in, but your approach here feels nicely fresh and from an electronics perspective - different! Keep up the good work.

[@NotMarkKnopfler]

Great vid thanks for covering memory decoding! I'm wondering how we would split off say a 1k or 4k or whatever portion of the memory map for io of some kind... Although admittedly on the Z80 you don't really need to do that since there is dedicated IN and OUT instructions. Although, if we wanted to have say 8k of ram, and the rest rom (for example) then we need to decode the address bus. That's where I get lost! Hope you cold gets better soon 🤧

[@DrBudgieandtheHumanMan]

Thanks mate! Ohhh right you want hardcore decoding. I'm not sure how a real engineer would do it, but this is how I would go about it. If I want the first 8k of memory to be RAM and the rest ROM: 1) log_2(8000) = 12.965... 2) the nearest whole number is 13 3) So I want the RAM to be selected up to address 0x1FFF (0001111111111111) and addresses 0x2000 (0010000000000000) and above to be ROM 4) So I'd OR together A15, A14 and A13 and the output of this would go to the RAM chip select pin and the inverse would go to the ROM chip select (assuming that the RAM and ROM had active low chip select pins). You can also OR the MREQ signal with A15, A14 and A13 to keep the IO space separate from the memory space. It gets a lot harder If you want to split the address space up further but it's a similar process, you just workout what address values you want your memory or IO device to be active between and use logic gates on the address bus to turn that combination of 1's and 0's into the chip select signals. You can get memory decoders which do some of the work for you and if it get really complex you could cheat and program a ROM to do the decoding for you. The address bus of the ROM connected to the Z80 address bus and then each bit of the data bus of the ROM goes to the chip select pins of whatever devices you're using. Hope this makes sense (a proper engineer would be able to explain this way better). Thanks, I'm slowly beating the cold! Haha