May I ask what IDE you're using for the '4809 code, please? If MPLABX, which version? The code is very interesting but swapping banks in and out of the $C000..$FFFF ROM address space for Flash write operations seems like a PITA

I use VS Code for coding and avrdude to flash the MCU. Changing ...

Looking forward to seeing the schematic.

Be careful what you wish for. :wink:

Schematics attached. And yes, I've redone the decoding part to stop people yelling at me.

Also yes, the 4809 is indeed a beast. 20MHz by default, loads of GPIO, single pin programming (UPDI). It's available in 40-pin ...

Something for you to consider with your next design.quote]

Thanks for the info. It is, in fact, a 4-layer board. And the size is fixed because this is a backplane design. The boards all have mounting holes in the same places so I can use standoffs to provide rigidity. (Pix attached.) There are four ...

Well, I may have been a bit previous. It works for certain values of 'work'. I can upload a new ROM image fine. An the 6502 machine reboots and appears to run. But certain programs are now crashing.

Some short user programs run without issues. Others crash, and always at the same point. Yet the ...

I'm not too happy with being asked to proof-read AI output for someone else's query.

Not sure what you mean by proofreading but I am sure that's not what I was asking.

We all know that people come to forums like this as a way of getting answers to problems they're having. That's wasn't what I ...

Here's an interesting conundrum. I have an issue with my new homebrew machine setup that uses flash (an SST39SF010) in place of an EEPROM. But I'm not (yet) asking for solutions to this problem. I want to explore it myself some more first because ... well, it's fun.

But I am interested in getting ...

In case anyone was at all curious, I got this idea to work.

I reworked the CPU board of my computer (it's a backplane design) replacing the EEPROM with an SST39SF010 flash chip and an ATmega4809 MCU. Now, when I want to update the ROM code, I simply push it from my dev machine over serial to the ...

There is also a (probably made for TV) movie with Derek Jacobi as Alan Turing. I watched it a long time ago, but I remember that I liked it better than The Imitation Game. A matter of taste of course, but it was clearly more historically accurate.

That was 'Breaking the Code'. I saw Jacobi do it ...

Thinking further about this, many of the STM parts have enough pins to represent an actual eeprom part; with an adequately fast part with sufficient RAM - many are 20kB or better - one might use the RAM for emulation and write to that ram via an external link (e.g. a nucleo board provides in ...

Oh, and I've just learned that STM does a middleware package called X-Cube-EEPROM that looks like it does exactly what I want, although this learning curve is rapidly getting steeper!

Instruction and data caches can be turned off on the STM32. And there might not be much of it going on. The program is going to be extremely simple:

* Check if /OE is low.
* If it is, get the address from the address port pins
* Put the relevant byte on the data pins
* Wait for /OE to go high ...

I'm sure you could make that work if the uC is fast enough.

My current candidate is the STM32H523CE, which can rattle along at 250MHz, has plenty of flash storage to hold the ROM image and enough GPIOs to handle both buses and the control signals with some left over. It’s also small enough that ...

you probably wouldn't want those three 74HC541's left in-circuit when switching back to normal 'run' mode operation.

Yeah, I am trying to reduce chip count! I currently have a board being fabbed that uses a modified version of my initial approach using an MCU (ATmega328PB) and a flash chip. I ...

The concern is the gate’s output rise/fall time, which is comparable to the 74HC equivalent and does not meet WDC’s specs for the clock input (5ns max). That may, or may not, cause some timing trouble, depending on how fast you plan to run this thing.

It's strolling along at 1MHz. When you say ...

Okay, just in case there's anyone still playing along at home, here's what I've arrived at.

This is the clock circuit:

Z64_clock_gated_mono.png

There's a new NAND gate (an AHC part with an average speed of 7ns). The two inputs are the ouput of the oscillator and a signal from an MCU GPIO I'm ...