[v2.0] MECB 68008 CPU Card

[Editor]

Some further progress with testing the 68008 board..

This is really awesome work! It's great that you've picked up the 68008 challenge, as I don't seem to have managed to get back to it (nearly 2 years later), with being seemingly constantly buried in other MECB related projects.

So, I think it sounds like you've officially picked up the 68008 reins!

Q. Have you brought out A16 - A19 to the Bus, to also allow linear address space allocation with the MECB 1MB ROM Expansion card? Or, is this not necessary? (e.g. if you have plenty of ROM space on the CPU Card itself)

I'm looking forward to hearing more. A truly awesome effort so far, especially with getting the M6800 Peripheral Control working on the Bus for the existing MECB Cards!

[epaell]

Oh, I completely missed that the ROM expansion had the option to connect the full address space to the bus - I had only previously used it via the switch selection and so assumed that it wasn't fully available - looks like all I need to do is reprogram the PLD. I do have all address lines going out to the bus (including the extra A20 and A21 that you get with the PLCC version of the 68008). Speaking of which, I had a comedy of errors in trying to get the plug for the Eurobus - I thought I ordered the correct plug but when it arrived I realised it only had two rows of pins ... so again I placed an order thinking I had the right one and it had three rows but with actual pins in the middle row ... I had to manually pull out those middle pins so that I could solder it on the board - it felt like pulling teeth!

I've set up the board with 0.5 MB RAM ($000000-$07FFFF, allowing expansion up to 2 MB continuous if needed via the MECB bus), 0.5 MB ROM ($200000-$27FFFF) and the I/O is mapped to $3C0000-$3CFFFF (it's wasting much more space than it needs to but I didn't have much room for finer granularity). During boot, it temporarily maps the ROM to the lower part of the memory space so that the RESET vectors can be accessed (upon reset the vector table can be modified in RAM - to allow redirection, using a trick I noticed on another 68000 board). I'm still not entirely confident with peripheral handling (I've only had to deal with 68000 devices on a 68000 previously - and that's much easier) and whether I'm handling the traditional-style interrupts and the MECB_MREQ signal correctly. So far things seem to be working fine with the ACIA, PTM and VDP and it seemed I was able to get interrupts working with the PTM. I made a whole series of blunders mixing up the active state of the ECB_RD and ECB_WR signals so I've got way too many bodge wires on the back of the board.

Anyway, your prompt above has given me an idea to test the expansion ROM in the $080000-$0FFFFF space (so it'll needs a little modification to the PLD) - that'll at least let me know if I have the MECB_MREQ signal working. I can easily test it without even having to overwrite what is already in there (It has a mix of 6809 and 6502 software) - I can just check via TUTOR to see if that data appears in the memory space. I could possibly add a hack to the expansion ROM board to connect up the upper address lines to shift it up to the same location as my current ROM - that would then allow me to perhaps replace the on-board ROM with another RAM. Although, at least for the time-being both seem to have ample space for messing about. If I ever want to run CP/M on there things might change - but there would be a lot of extra work to get to that point (and I'm not really clear as to how usable CP/M is on that platform anyway). The main thing the system is missing at the moment is somewhere to load/save things. Anyway, it would be nice to confirm whether or not the board is playing nice with the MECB_MREQ signal.

BTW, TUTOR has been great for debugging - it's really nice being able to load, set breakpoints and single-step to see what is happening (it really paid off having those early MAME experiments as it made the software modifications for this board quite simple).

[epaell]

I mapped the external expansion ROM into the $100000-$1FFFFF area and that seems to work. On the left is a terminal dump from TUTOR of part of the ROM area and on the right is part of the ROM contents in a hex editor. So it seems to be reading the data correctly. It's been a while since I last programmed those ROMS so I couldn't quite recall exactly what I put on there but it definitely seems to have that same version of BASIC.

I realised that I didn't actually connect that part of the PLD to the bus ... sigh, so more bodge wires (I think at the time I was planning for it but thought I was being ambitious enough just trying to get the onboard RAM and ROM working). I'll have to make a new board with all the fixes (hopefully without introducing more errors). I'd also like to rearrange the PLD signals a bit to make them a bit more logical and efficient - there was a bit of guess work in the original test run and now that I have something operational it would be good to clean up. However, with so many distractions I suspect this may require further iterations in future.

[epaell]

Ran a more thorough test by programming both ROMS in the expansion card with a sequence of values and then read them via a simple 68008 program and it confirmed that all of the data is readable. I also confirmed that the FLASH commands work on this ROM (something that was a bit tricky to use when the full ROM wasn't present) i.e. I was able to read the manufacturer and Chip ID. Also confirmed that this doesn't work if the ROMs are write-protected using the jumper. So thankfully all working as expected - phew!

[djrm]

Very nice to see your 68k system taking shape. I have a couple of dip 68008 chips around here somewhere, maybe some prototyping going to happen here soon.

I felt your pain with the ordering of the wrong connectors, I have ten double row ones in the naughty corner here. Almost as bad as soldering the correct ones in back to front, which I have done more than once - in fact the last time was just a few days ago. The silk screen is very clear, I don't know how one of the four got reversed - must be more careful...

Looking forward to seeing what else you can get running on you board. Best regards, David

[Editor]

I do have all address lines going out to the bus (including the extra A20 and A21 that you get with the PLCC version of the 68008).

Awesome. I was just originally intending to go with the DIP48 version of the 68008 (hence 1MB), but I've certainly taken a renewed liking to PLCC packages over the last couple of years, and the extra A20 & A21 lines (and also A22, A23) are identified on the bus pin-out for further address bus expansion.

Ran a more thorough test by programming both ROMS in the expansion card with a sequence of values and then read them via a simple 68008 program and it confirmed that all of the data is readable.

Nice!

I note your comment about CP/M. Interestingly, my own personal driver for MECB related projects is as a lower level exploration of the early microprocesssors and peripherals. i.e. Not necessarily for building more comprehensive "higher level" systems (although people certainly could do this with the MECB platform).

In other words, for my own personal 68008 desires, I'm more interested in exploring the 68000 Assembly Coding experience on the original hardware (via a good Monitor), and hardware interfacing etc., rather than via a "higher level" Operating System (like CP/M).

But having said that, getting any original microprocessor system up and running is a good challenge, and more importantly FUN!

[Editor]

Almost as bad as soldering the correct ones in back to front, which I have done more than once - in fact the last time was just a few days ago.

Don't feel bad. I can freely admit that now, on 2 seperate occasions, I've soldered in a right angle Female bus connector on a new MECB Card (instead of the Male connector). LOL

Sometimes I think the brain goes into auto-mode, and you just select the wrong component when you concious mind is elsewhere (thinking about the next thing).

I'll even admit that on one occasion, desoldering the wrong connector and soldering in the correct Male connector, actually broke one of the PCB traces without me realising. This then cost me a good number of hours fault-finding, what I first suspected was a design error failure.

But, we all enjoy this hobby for the challenges it gives us!

[epaell]

In other words, for my own personal 68008 desires, I'm more interested in exploring the 68000 Assembly Coding experience on the original hardware (via a good Monitor), and hardware interfacing etc., rather than via a "higher level" Operating System (like CP/M).

I think my only reasoning for wanting to have something a little bit more is just so that it could be like a complete system and not have to connect up with my laptop to write/upload programs. Having said that, it's probably a lot easier this way anyway since I can use a nice editor and compiling is almost instantaneous (and the upload is not that slow). I am loving the 68000 Assembly Coding experience - it is such a HUGE step above the earlier generation of processors. I'm currently writing up various support routines for the VDP and FLASH - once I have those in ROM then the actual overhead in uploading new programs is much smaller (the worst is having to upload the font data each time). There is heaps of space in the onboard ROM (even with TUTOR and enhanced BASIC in there) and now I have the extra 1 MB available in the expansion ROM too. I think I'm going to enjoy playing with this particular system, though, I think my next step is to clean up my design and update it in GitHub because the current one is embarrassing (in case anyone makes the mistake of thinking it is final). Once I can verify a final design then I'll feel a lot better.

[epaell]

I felt your pain with the ordering of the wrong connectors, I have ten double row ones in the naughty corner here. Almost as bad as soldering the correct ones in back to front, which I have done more than once - in fact the last time was just a few days ago. The silk screen is very clear, I don't know how one of the four got reversed - must be more careful...

Oh no! My other frequent error is soldering the IC sockets in the wrong way around - again, a really careless error given that it is so clear in the silk screen which way they should go but thankfully still workable (though it then requires greater care when inserting the devices to ensure they match the silk screen and not the socket).

[djrm]

Whenever I see a nice looking white marker pen I'll buy it. I use them for adding clear markings for connector pin one and such on boards (top and bottom), usb connectors too so you know which way to plug them without the usual trial and error fumblings. I wish I could say it stops me making stupid mistakes.