MECB 6802 CPU Card

[PaulShipley]

Hi David, I haven't tried any of this, but my thinking was that modern devices can run at much higher clock speeds than the 6800, so using a PLD with (say) a 64MHz clock you could easily divide the 1MHz required into slices (no RC network required). Similarly, with an MCU (eg PIC), a fast clock would allow precise GPIO signals to be used as the 6800 clock. Of course, this is just my speculation as to how I would try to implement a 6875 equivalent - though still more achievable than finding a genuine used working one.
It would probably be possible to create an oscillator with a PLD, though I was thinking of an external clock. Oscillators are a "solved problem", and oscillators failing are still a major cause of circuit failures. I wouldn't try to reinvent this.

[Editor]

Just thought I'd mention that I did buy a handful of original MC6875 chips a few years back from AliExpress (for getting my original DREAM 6800 back up and running, and other genuine 6800 projects).

I just checked, and although from a different seller, there does still appear to be verified MC6875 chips available from AliExpress.

Take a look here: https://www.aliexpress.com/item/4000045848422.html

[djrm]

I just checked, and although from a different seller, there does still appear to be verified MC6875 chips available from AliExpress.

Thanks Greg, I'll get one to go with my 6800 chip - I see free postage on it next month too.
Best regards, David.

[Editor]

I imagine implementing the clock delays introduced by the RC networks in the circuit Greg posted would be impossible.

The MC6875 and the substitution circuit does interest me.

My curiosity is due to the datasheet specifications of the M6800 dual phase clock. Specifically that the timing of the "non-overlapping" 2 phase clock, indicates a "Delay Time or Clock Overlap" (Td), of Minimum 0ns.

i.e. No seperation of the "non-overlapping" phases appears to be actually necessary? (just no overlap).

Looking at the block diagram of a current 74HCT74 flip-flop, it appears that there is no propagation delay between the non-inverted Q and the inverted Q outputs, which makes me wonder if any RC delay is actually necessary?

I note also that the (substitution circuit), implemented RC delay, would affect both the rising & falling edges of the phase clocks (leading to an overlap at one end?).

i.e. Would it work if we simply drive the phase 1 & 2 M6800 clock inputs directly from a 74HCT74 dual flip-flop's divide-by-4 non-inverting & inverting congruent outputs?

Of course, I'm sure this would have been tried before (and, no doubt, my hurried thinking is overlooking some important subtle timing detail).

[epaell]

In case it is of interest to anyone, I can confirm that the little 8-pin PIC alternative to the 6875 works fine with the 6800 I had (which I think was made in 1977). I made a quick prototype board to check it out. I haven't been able to get SWTBUG working on it yet for some bizarre reason but I did get a version of STAR TREK running (given the 1977 date perhaps Star Wars would've been more apt?) - which brought back a flood of childhood memories! I had one minor error on the board and after looking at the PIC more closely I realised that there is also reset circuity there so I think I could reduce the chip count by one.

[epaell]

I was able to remove the reset circuitry (that used the TC1232CPA) relying solely on the PIC to do both the RESET and clock control and this seems to work fine as well. I also managed to get MIKBUG going ... actually, I'm kind of surprised anything worked before because it turns out my RAM chip didn't have +5V connected (it seemed like the simplest thing and I thought this was the first thing I checked but it caused a huge amount of confusion when I was debugging because the RAM seemed to partially work but would give very odd values at times).

I'll have to build up the keypad and see if I can get the DREAM 6800 software running in its original form (with the OLED display extensions). I'd also like to finalise the design so that all the bugs are fixed in the final board. A project for the New Year methinks.

[Editor]

I'll have to build up the keypad and see if I can get the DREAM 6800 software running in its original form (with the OLED display extensions). I'd also like to finalise the design so that all the bugs are fixed in the final board. A project for the New Year methinks.

Nice work!

It’d be interesting to see the DREAM running on original 6800 family hardware.

It should be relatively easy to take the original CHIPOS code and switch to the OLED display extensions. At least avoiding the need to convert to 6809.

It will be interesting to see how it performs. Although even at 1MHz you’re immediately gaining 40% performance, by not having the DMA driven Display.

This should be more than enough performance to compensate for the OLED pixel updates overhead.

Alternatively, you’ve also got 1.5MHz and 2MHz options up your sleeve.

You’ll also be able to run DREAM Invaders with a much simpler conversion.

[epaell]

Managed to get MIKBUG 2 running on the 6800 board and on top of that a highly modified CHIPOS (I don't have a keypad so I hacked in ACIA input instead and also use the 76489 for the beeps and bloops) ... but fell for the old stack-works-differently in the 6800 compared to the 6809 (and so had to undo some of the fixes that were put in for the 6809 version) ... and also struggled a bit by the lack of a PSHX/PULX/ABX operations (which required some creative work-arounds).

https://www.youtube.com/watch?v=4Klg9vusHwg

[Editor]

Managed to get MIKBUG 2 running on the 6800 board and on top of that a highly modified CHIPOS...

Really nice work!

Q. Is the speed (on the video) running at 1MHz, or did you bump it up tp 2MHz?

If 1MHz, then that looks great! I'm just guessing that the overhead of the OLED pixel based updates, may be mirrored by the 40% performance gain achieved by the absense of the original display refresh DMA.

If so, your 6800 version makes for an even more accurate re-creation (as-in, less changes to original 6800 machine code programs / routines will be required)!

Perhaps a relatively simple DREAM Invaders conversion is next (if not already done)?

[epaell]

Yes, I believe it is just a standard 1 MHz clock (although I haven't actually confirmed that the PIC accurately generates this) - I'll try check it which a CRO tomorrow.

I have converted and compiled DREAM Invaders but while doing so I realised that I really needed a keypad for it to work because it looks for keys down whereas the ACIA input doesn't quite provide that kind of signalling. Once I get back from holidays it'll be my next fun thing to look at ... and sending off the final board just to clean things up so I can get back to 68008 goodness.