MC68HC000 seems to break STE programs?

[Cwiiis]

I bought the MC68HC000 CPU from the exxos store in the hopes that it'd be an easy fix for DMA problems on the STE. On replacing, everything seemed fine, but with it in, no STE software starts up. Either it complains about lack of RAM (though RAM tests show all 4MB working fine) or it just drops back to GEM immediately after launch. Putting the original CPU back, everything works as expected.

I was under the impression that it should be a straight drop-in replacement, but is there some other process that I'm missing that needs to be applied?

My machine has exxos 4MB RAM, the exxos STE TOS switcher and a replaced floppy drive, but is otherwise stock.

[dad664npc]

It is a drop-in-replacement. I have recently swapped an HC part in for my suspect STe (ACSI bus write problems - aka bad DMA). All is good with the HC part for me. Give a visual inspection of the HC pins to make sure all are at the same level - could just be one or more aren't making proper contact. The other thing to try is to clean the socket with an alcohol cleaner. Failing that it could of course be a bad CPU

[Cwiiis]

I haven't performed the bus resistor replacement and from a bit of reading, it sounds like this may be more necessary when replacing the CPU - viewtopic.php?f=17&t=2235#p27504 - Given it's mostly good, I'll continue with my old CPU for now but will likely give this a go when I'm next doing some other mods on the machine (I intend to replace the PSU soon). I'll report back if/when I do that.

[exxos]

Have you let it verify ram on boot fully ? Ran YAART etc ? Give the ram a wiggle in the sockets in case there is a bad connection.
Also make sure you have no bent CPU pins etc.

What revision motherboard do you have exactly? A image would help.

Atari did various modifications to the STE including resistor pack changes even before doing my mods.

[Cwiiis]

Have you let it verify ram on boot fully ? Ran YAART etc ? Give the ram a wiggle in the sockets in case there is a bad connection.
Also make sure you have no bent CPU pins etc.

What revision motherboard do you have exactly? A image would help.

Atari did various modifications to the STE including resistor pack changes even before doing my mods.

I've verified the RAM with the original CPU, not with the HC CPU. I've not run YAART in either configuration. I don't think there are bent CPU pins, but I'll take a look (and I'll perform the tests too). I've attached a picture I took earlier, apologies for the PSU being in the way. It seems to be identical to your STE picture in the bus resistors thread.

The machine runs well, except what I believe to be rare, intermittent DMA-related issues. Every now and again (increasingly rarely, it seems) a folder I navigate to on the ACSI2STM appears blank. I have a PSU on order, this one (which has been serviced, but as you can see is a little the worse for wear, with a crack in it which has been patched underneath the board) runs worryingly hot. I don't want to put too much wear on the CPU socket when it's basically running as well as I need it to for 99% of what I want to do on it, but I will change out the resistor networks and double-check the CPU once I receive the new PSU.

[tzok]

ICs from different manufacturers may have slightly different pin profiles, thus a new CPU may not have good contact with a socked in which another CPU has been sitting for the last 30+ years. When swapping a CPU for another, you must use Deoxid or a similar product on the socket, especially because sockets used by Atari are not of particularly high quality.

Also, the POST RAM test is not particularly thorough, you really should run YAART in the affected configuration, especially because ST titles usually required only 1MB of RAM, while the STe ones are those which may require 2 or 4MB, and that may be the reason for crashing.

[Cwiiis]

So I've since learned that the game launcher I was using disables >1MB RAM - I swapped back CPUs and actually, the HC variant is completely fine. Sorry for the noise!

If anyone knows of a program to re-enable the RAM, that'd be appreciated - this RAM disabling persists across resets, so the only 'fix' is to turn the machine off for a while. I've searched around a bit, but can only find programs that set the machine up as 512K or 1MB, but not programs that reverse the operation. Bit annoying if I fancy playing a game and then playing something STE enhanced, or doing some work in Cubase (NVDI then takes up too much memory to use without more than a meg...)

[exxos]

Doesn't a cold boot via keyboard clear it ?