Understanding Support ICs for MCU

[chickenHeadKnob]

Okay I would like to side-step the Y problem and offer a glimpse at one possible solution to the X problem. Have a look at the web page on the Commodore KIM-1.:
https://www.commodore.ca/commodore-products/productscsg-mos-commodore-kim-1-history-pictures/
It is well documented and has all the manuals and schematic. This was a well loved little starter computer of the 70's. I am not saying you need to slavishly copy the KIM-1, I am simply suggesting you could study the design and then come back here with questions on specific aspects you don't understand. While 7 segment displays look more complicated than a single led and are more involved software wise if you want to display coherent numbers in reality they are just a bunch of single leds with a common anode or common cathode connection. So lighting up one segment as an indicator is just the same as a single led.

[David Hess]

Also I suspect inexpensive simple programmable logic could be used to aggregate the decoding logic with an I/O register, so one side connects to the microprocessor bus and the other wide provides input and output bits.

That was done on at least a few PCs back in the 90s. IIRC the Mac SE was one of those, it was more or less a refined version of the earlier 68000 compact Macs with the address decoding circuitry replaced by some PALs. A modern-ish 5V CPLD would work well for this.

At some point after the IBM PC AT came out, discrete decoding logic started to be replaced with PALs, but I was referring to making the whole I/O port including the decoding logic in a CPLD or similar for a single chip solution.

[james_s]

Well you certainly could, I've made entire computers in FPGAs, including custom decoding and IO.

[wayfarer.technologies]

MPU + support ICs were the way to go before integration migrated those functions onto the chip, thus creating a MCU.

If you want make a functional thingy, then the get there fast with a modern MCU.

If you want to learn how in the past all the parts were combined so that a MPU could deal with the outside world, then by all means continue your learning experiments.

Here's somebody to did that to a 6502 + support ICs to make a familiar hardware experiment platform: https://hackaday.io/project/190260-65uino A lot of people do this kind of project out of nostalgia or just for kicks.

so the 65816 needs demultiplexing, there are things you can do with the clock and I think a flip-flop for a "really clean edge" and some places adding a few passives can help fix a bug, I mean feature 

generally this is education, though I am planning to use the 65xx series for some projects. I have concerns with several mcu offerings out there, largely due to a constrained memory access to what is 'on-chip'. I do plan to use a modern mcu for some sort of "board controller" or "RIOT chip" (RAM/ROM, I/O and Timer). I intend to pick 1-2 architectures and stick to them, and  the 65xx is as good a choice as any. For mcu, I am looking at the msp430 (which I hear is on its way out) and the PIC series, which I hear is moving to MIPS, so I might use that instead.
in the near future I am most likely to just order a set of 74000 and 4000 ICs and blink LEDs while following along with youtube videos and some exercises. even watching those is helping a lot with understanding what is going on before I start getting into things like building a calculator, "gameboy" or pick up on Lee Davison's work and build a 6502 based plotter.

I am trying to pick back up on the basics before I go forward.

Also I suspect inexpensive simple programmable logic could be used to aggregate the decoding logic with an I/O register, so one side connects to the microprocessor bus and the other wide provides input and output bits.

Some microprocessors and microcontrollers had support I/O chips available which did almost everything except some of the decoding.  The 68HC11 series had the 68HC24 (port expander?).  Intel processors had the ubiquitous 8255 which is better known as providing the parallel printer port capability of early IBM PC type personal computers.  The 8255 and similar chips can be adapted to other microprocessors without too much difficulty:

https://www.renesas.com/us/en/products/space-harsh-environment/mil-std-883-products/mil-std-883-microprocessors-and-peripherals/82c55a-cmos-programmable-peripheral-interface

Also I suspect inexpensive simple programmable logic could be used to aggregate the decoding logic with an I/O register, so one side connects to the microprocessor bus and the other wide provides input and output bits.

That was done on at least a few PCs back in the 90s. IIRC the Mac SE was one of those, it was more or less a refined version of the earlier 68000 compact Macs with the address decoding circuitry replaced by some PALs. A modern-ish 5V CPLD would work well for this.

I may get into CPLDs or PAL/PLA later. not quite ready to jump into those yet.

Okay I would like to side-step the Y problem and offer a glimpse at one possible solution to the X problem. Have a look at the web page on the Commodore KIM-1.:
https://www.commodore.ca/commodore-products/productscsg-mos-commodore-kim-1-history-pictures/
It is well documented and has all the manuals and schematic. This was a well loved little starter computer of the 70's. I am not saying you need to slavishly copy the KIM-1, I am simply suggesting you could study the design and then come back here with questions on specific aspects you don't understand. While 7 segment displays look more complicated than a single led and are more involved software wise if you want to display coherent numbers in reality they are just a bunch of single leds with a common anode or common cathode connection. So lighting up one segment as an indicator is just the same as a single led.

my long term project may involve building something like a KIM-1, though I am certainly focusing on my own design rather than that one specifically. right now, I am working on a few building blocks before I move further.