STE glue/mmu problem.

[hildenborg]

So my STE died while I was doing some experiments with it (bad me!)
I guess that the most probable cause is that I accidently shorted something without noticing.

I have found the problem to be that the OE signal that connects the GLUE/MMU with the Shifter and the MFP is always low.
I checked and it is not shorted, so nothing is actively pulling it low.
Apart from that, the MMU/GLUE seems to be operating as it should...

So my question is:
Does the MMU/GLUE need other signals to be active in a certain way to enable the OE signal, or can I just assume that it is broken?

The chip in question is: "C300589-001".
It is the variant without blitter in it.

[ijor]

I have found the problem to be that the OE signal that connects the GLUE/MMU with the Shifter and the MFP is always low.

Where do you see such OE signal at the schematics??? Which pin number at each of those chips?

[hildenborg]

Where do you see such OE signal at the schematics??? Which pin number at each of those chips?

My bad... I read wrong on the schematics, it says "DE" not "OE", which makes sense as it is "Display Enable"... Sorry.
It connects MFP pin 20 to MMU/GLUE pin 141 and to Shifter pin 54.
I think it is the signal that generates the Timer B interrupt on the MFP.

[ijor]

So my STE died while I was doing some experiments with it (bad me!)
...
My bad... I read wrong on the schematics, it says "DE" not "OE", which makes sense as it is "Display Enable"... Sorry.

I see. But the machine should still work without DE. You won't get any display. But it should still boot. You should still be able to see disk activity and even the monitor should still sync. If the machine is actually dead, not just that you don't get video, then there must be something else.

Anyway, if DE is not working at all that means that either the MCU (the MMU/GLUE combo) is failing, or the signal is somehow failing at the board level.

[hildenborg]

I see. But the machine should still work without DE. You won't get any display. But it should still boot. You should still be able to see disk activity and even the monitor should still sync. If the machine is actually dead, not just that you don't get video, then there must be something else.

I thought so too, but what happens is that somewhere in TOS ROM (2.06 Swedish), there is a loop that waits for MFP timer B to change. Which doesn't happen without a working DE signal. It was that deadlock that first made me aware of a possible problem.

Well, I've ordered a new MMU/GLUE.
So next fun thing: replace a 144 pin smd chip

[Steve]

First thing you should do before ordering parts is simply clean the socket with deoxit/contact cleaner.

[ijor]

I thought so too, but what happens is that somewhere in TOS ROM (2.06 Swedish), there is a loop that waits for MFP timer B to change. Which doesn't happen without a working DE signal. It was that deadlock that first made me aware of a possible problem.

Ah, yes. I forgot that a PAL TOS would wait for the vertical blank when changing the frame rate from 60 Hz to 50 Hz after reset.

[hildenborg]

First thing you should do before ordering parts is simply clean the socket with deoxit/contact cleaner.

No socket. It is a 144 pin smd chip that is soldered to the mainboard.
And I have measured to make sure that there are no bad connections.

[ijor]

No socket. It is a 144 pin smd chip that is soldered to the mainboard.
And I have measured to make sure that there are no bad connections.

Did you verify that MCU chip is actually the problem, and not something else that is shorting or pulling down the signal?