MEGA ST2 - "random" characters on the screen

[DominoTree]

I swear I'm not losing my mind - I have a new bone-stock MEGA ST2 that also does the exact same thing. From what I can see it looks like something is putting signals on the address bus at around 2.8V and the ACIAs use A0 and A1 as enable lines and the low-voltage signals don't seem to be triggering them properly. The datasheet *says* they should register 2.0V above Vss as "on" but that's not what I'm seeing.

Later today I'm gonna see what happens if I drop some MOSFETs in front of them on those two lines to act as switches to try to feed them a cleaner signal at 5V.

On a semi-related note, switching to an XC68HC901 seemed to clean up a lot of signals pretty well and otherwise works as expected. I've also got some MC68B50s on the way to see how a faster component behaves. Experiments!

[exxos]

I swear I'm not losing my mind - I have a new bone-stock MEGA ST2 that also does the exact same thing.

2 odd faults on 2 different machines like that.. madness.... Though you could just slap like 1K resistor pullups on such pins to "help" it hit the proper logic levels.

[exxos]

Another thought.. have you checked the DMA control lines ? as I recently found the DMA was corrupting the bus during reset. So if one of its control lines has a bit of noise on it, it could enable it when its not supposed to be and corrupt the bus...

[DominoTree]

I need to dig into the DMA more anyways - tried to get TOS 2.06 going with the decoder on my FrankenMEGA and I could only get it happy by switching all of the address/data bus pull-ups back to 10k and removing the DMA bus pull-ups entirely (really glad I socketed all of this stuff lol)

68B50s dropped in and worked fine but the character issue still remains.

I did have one thought - characters coming in from the ACIAs also trigger sound to be played - simply pulling the YM2149 and the (I think) 74LS02 driving it didn’t change the behavior but that whole process involves a few other components I haven’t looked at too closely.

At this point as long as I’ve got a keyboard plugged in there aren’t really any issues with the system, and just adding a 10K pull-up on the RX line keeps it from spamming phantom keypresses and screaming if there’s no keyboard attached, so I’m really just satisfying my own curiosity

I can see why everyone is focused on building new hardware versus figuring out all the quirks of these old systems!

[exxos]

I can see why everyone is focused on building new hardware versus figuring out all the quirks of these old systems!

Exactly. These bizarre issues are coming in faster and faster as time goes on. I just couldn't keep on top of diagnosing them all, it was a full time job. Would be good to know what is causing all these problems, but the mandatory fixes only cover all the issues I have currently had and diagnosed. Mostly easy fixes, but the work which goes into it all.. well you can imagine Most of the problems are down to bad PCB layout, unfortunately the MEGA ST board is the worst boards out there. Its why I suggest the 70789 style boards as they are the "best" ones. In the end (at least for me) I just admitted defeat and with the help of this community, we created a new board to do it "right" from the start.

[DominoTree]

I can see why everyone is focused on building new hardware versus figuring out all the quirks of these old systems!

Exactly. These bizarre issues are coming in faster and faster as time goes on. I just couldn't keep on top of diagnosing them all, it was a full time job. Would be good to know what is causing all these problems, but the mandatory fixes only cover all the issues I have currently had and diagnosed. Mostly easy fixes, but the work which goes into it all.. well you can imagine Most of the problems are down to bad PCB layout, unfortunately the MEGA ST board is the worst boards out there. Its why I suggest the 70789 style boards as they are the "best" ones. In the end (at least for me) I just admitted defeat and with the help of this community, we created a new board to do it "right" from the start.

Glad to be helping on that front - even if it is just Icky’s new keyboard for now

[DominoTree]

On that note maybe I should get back to work on that instead of MEGA rabbit holes

[exxos]

On that note maybe I should get back to work on that instead of MEGA rabbit holes

Do both, multitask