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CPU Strangeness

Posted: Sat Apr 27, 2019 4:58 pm
by rubber_jonnie
I currently have 2 'dev' boards that I use for various test and beta hardware projects, The came to me as bare boards, so I wasn't so worried about using them for this function.

The first board, a C103253 is a bit odd in that it has SMT RAM, but otherwise a sound board, and I removed the CPU so I could fit a socket and use it for testing my PAK build, and subsequently the IDE interface we're testing right now.

It has this CPU:
IMG_20190427_162227.jpg
IMG_20190427_162227.jpg (343.2 KiB) Viewed 784 times
The second board is a CO70789 REV D. Since I wanted to test the IDE interface on another board, I removed the CPU from this board too, and fitted a socket here too.

It has this CPU:
IMG_20190427_162235.jpg
IMG_20190427_162235.jpg (325.42 KiB) Viewed 784 times
Now here is where it get's odd.

Since the IDE board already had the CPU from the C103253 already fitted to it, and not wanting to start prying the CPU out of the IDE board, I figured I'd plug it straight into the CO70789. When I powered on, it was like the board had no CPU installed!

At first I though it was something I'd damaged during the socket installation, so I ended up removing the IDE board, and the CPU from the IDE board.

At this point I had the CPU from the C103253 in my hand, so I plugged it into the CO70789, thinking nothing of it. Still the same, it was like there was no CPU installed.

WTF!

So I pulled the C103253 CPU from the CO70789 board, and fitted the original processor back in. Lo and behold it powered on just fine.

I then pulled the CPU from it's socket, fitted it to the IDE board, plugged it back in and all good.

So basically, my CO70789 board would not work with the CPU from the C103253 board.

So it seems there are differences between the two processors, though what they are I don't know. Well, yet at any rate, I'm definitely going to have a look later!

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 5:18 pm
by exxos
Yep pretty normal screwyness. This is one reason I only stick to later boards. Aside from wiping all pins with IPA etc. You can do the same with a HC CPU...

I've also had issues just moving the cpu on a adaper board on early booster tests where the couple cm track length was enough to screw it all up.

Finding the issues can take a long time, mostly I gave up and just stuck to the later boards. Though changing the bus pull ups to 4.7k is likely a good start.

Also even the loading in the 8mhz clock can screw things up. I had to put a 47pf on it on some booster boards else it wouldn't boot at all without it.

More motivation to just create a new motherboard ;)

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 5:37 pm
by exxos
BTW.. Some revisions even suffer from DMA issues :lol:

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 5:40 pm
by PhilC
I'm sure we can make you a BAD CPU sticker if that helps?

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 6:15 pm
by exxos
PhilC wrote:
Sat Apr 27, 2019 5:40 pm
I'm sure we can make you a BAD CPU sticker if that helps?
Bad motherboard be more like it. Can sticker the whole lot then,

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 6:24 pm
by PhilC
exxos wrote:
Sat Apr 27, 2019 6:15 pm
PhilC wrote:
Sat Apr 27, 2019 5:40 pm
I'm sure we can make you a BAD CPU sticker if that helps?
Bad motherboard be more like it. Can sticker the whole lot then,
I guess I can do it in 60mm wide stickers? might be quicker to grab a paint brush?

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 8:41 pm
by rubber_jonnie
exxos wrote:
Sat Apr 27, 2019 5:37 pm
BTW.. Some revisions even suffer from DMA issues :lol:
No way :dizzy:

Maybe just get one big sticker that says bad ST......

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 8:49 pm
by PhilC
But seriously, are there any revisions we should avoid and any that we should go look for?

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 8:59 pm
by rubber_jonnie
Personally, my favourite mainboard is the CO70789.

Even then there are variations though, so there is an element of pot luck, and in my experience, every board has its made on a Friday examples that just don't follow the rules.

Re: CPU Strangeness

Posted: Sat Apr 27, 2019 9:16 pm
by PhilC
That's the one I'm liking the most so far. Got a couple of them so far, one more just last week.

Still a Mega ST fan tho.