[v1.1] MECB 8088 CPU Card

[Editor]

The ceramic devices really are beautiful!

My first CPU was a purple ceramic MC6800. I paid a fortune last year trying to source an equivalent purple CerDIP MC6809 from eBay (for my MECB 6809 CPU), but unfortunately it arrived non-functional, so I sent it back for a refund!

I also started PC programming with Pascal (back when it was still called "Turbo" Pascal) ...

Yes, indeed, it was of course "Turbo Pascal" back then. I still have my original V2.0 manual. Along with later versions, after many version upgrades.

... for some reason there is an interrupt generated when I first start up the board but it goes away if I wait long enough. My dodgy fix was to introduce a memory check at the start in the monitor software and by the time that finishes the interrupt goes away. It's a very dissatisfying oddity but I can't work out why it is happening. ...

Sounds like an interesting issue. I wish I had some Intel hardware experience to be able to help with this (these are the type of issues I like debugging!).

It almost sounds like something could be pulling down the interrupt line (slow rise time?), beyond the reset period? Have you scoped it?

BTW, while looking up a few other things (in particular how one might conceivably connect a C64 SID chip) I found that there were some nice little inverters for converting voltages and it got me into thinking that maybe an 8080 option might still be on the cards (pun intended)

What voltages need converting? I'm usually just dealing with 5V <-> 3.3V logic interfacing.

Unfortunately the connecting of the SID chip might be more complicated than I would've liked as it requires a 1 MHz clock (and that is used as part of the data transfer logic) - so if it is an independent clock it might cause timing issues with whatever MECB processor is being used or if the system clock is used it may not necessarily be 1 MHz (and/or I'm not sure if the timing would be correct).

Sounds like a potential perfect match for a 68008 running at 10Mhz (creating a 1MHz MECB bus clock).

[epaell]

Oh no, that must have been so disheartening i.e. non-functional ceramic 6809

I should really connect the scope to the 8088 board but haven't really had much time recently (I had a quick look at the levels and they seemed fine). Once I get an opportunity I'll have a closer look because despite the board otherwise working it is something that is really frustrating me!

The C64 SID chips require an additional +9V or +12V (depending on the variant of the SID). Similarly, the 8080 additionally requires +12V and -5V (fortunately at relatively low currents).

I've been somewhat drawn to the SID chips mainly because they have nice control over PWM and individual envelopes for each of the audio channels, and has filters ... also because there are some modern variants (given that originals are harder to come by). As a result of this it is able to produce some really nice sounds (at least compared to the AY-3-8910 ... which is not that much more capable than the SN74689, it only really add an envelope function but that is forced to be the same for all channels).

Good point regarding the 1 MHz bus with the 68008 - I'll have to see if I can get my hands on a SID just to play with. MECB prototyping board to the rescue

[epaell]

In my usual state of carelessness (but another case of why the heck did this work at all) I was looking at a possible Z80 card but when dealing with the system clock I started to wonder what did I do on the 8088 card ... only to find out that I must have had the same question there and never quite got to answering it So I passed the SYS_CLK signal into the PLD but never took it out again i.e. the ECB_CLK line was left hanging. I guess it would have been obvious that it was missing if I tried to use the timer on the Motorola board ... but I was left confused as to why the ACIA seemed to work fine. I'm not sure if this is somehow related to the initial spurious interrupt problem. I've manually connected the ECB_CLK to one of the spare output pins on the PLD and passed it through internally by reprogramming the PLD. I've confirmed that it hasn't broken the existing functionality but I'd need to reprogram the FLASH ROM without the memory check to see if the interrupt still occurs. Perhaps I should first confirm whether or not the timer works (since that uses the clock) ... and/or the SN76489 since it too uses the clock.

[djrm]

Hi, I have been having clock fun and games today, I now have a 1MHz device takling on a 4MHz 6502 cpu bus using a PLD to extend the chip enable by pulling the RDY line low.

[epaell]

OK, connected up the system clock to the bus and that works fine with the ACIA but I think it is now having exactly the same issue you describe i.e. the 8 MHz 8088 clock is too high for the TIMER. I replaced the 8 MHz clock with a 2 MHz one and it all works fine (well, at least the timer behaves as expected and generates interrupts on a regular basis). I haven't removed the memory check / delay from my monitor to see if that initial interrupt goes away with the clock now present.

In hindsight, maybe I should've used the 8284 clock generator - I think it has the ability to introduce wait states for I/O. Or maybe I just need to divide down the clock?

[epaell]

I did a really simple clock divide-by-two using the PLD flip-flop and that seemed to solve my TIMER problem. It's still over-clocking the timer by a factor of two but at least it works now.

[epaell]

Oh, interesting, I just updated my monitor to skip the memory check (AKA fudge delay to side-step the initial "spurious" interrupt) and the spurious interrupt has gone away. So two things have changed - I'm now powering the MECB with an external power source (rather than via the USB-C) and I have the divide-by-two clock going to the ECB_CLK. Unfortunately I can't quickly check with my old power set up because I inadvertently left my USB-C power cable at work (I need to remember to pick it up again next week). I guess I could go back and revert the full 8 MHz clock to see if it was that which caused the problem.

[epaell]

OK, so reverted to the old clock set up and the problematic interrupt doesn't occur (and, of course, the Timer doesn't work either). So I think that points to the USB-C not being able to provide enough current to feed the hungry beast. I can confirm this for certain once I get my cable back again.