MD hangs after playing any PCM file

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prophet36
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Re: MD hangs after playing any PCM file

Postby prophet36 » Mon Apr 17, 2017 1:19 pm

One thing I don't understand is if the rapid-switching of the buffer conveying the address-lines is causing /OE to also switch, that implies that /OE is not being actively driven. But if that were the case, then putting a 4K7 pull-up to +5V on /OE on the MD side would fix the problem, but when you tried that, it had no effect. I guess another thing we could try is to bypass the level-converter channel conveying /OE to the FPGA, and use a voltage-divider for it instead.

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Re: MD hangs after playing any PCM file

Postby amushrow » Tue Apr 18, 2017 10:33 pm

So, we can use diodes to reduce the voltage to the buffer, and in my test on a breadboard it seemed like it would work fine. But it doesn't, as I suspected the current is so low it leaks through anyway.

With a 2.7v Zener diode the 5v from the Mega Drive is dropped to 3.5v (a ways off 2.7v). The 1.5v signal which seems to be causing the trouble gets dropped to a 1.3v.
The result is that the noise has been damped somewhat, but it still happens and the Mega Drive still crashes.

Possible solutions include adding a resistor to ground along with each diode, which would require the PCB to be redesigned (making space a for a teeny little diode above each address pin would have been super easy). With that you could also remove the buffers, the 5v from the MD would be dropped to safe levels for the FPGA (although I'd probably try for a lower zener voltage, at 2.7 it would only leave 0.3v above the 2.0v required to guarantee that things will see it as HIGH). The only thing I don't like about this idea is soldering 48 piddling little components, I got some SOD-523 diodes so I could squeeze them in at the top of the cart pins and I swear some of them shifted to another dimension before I got to solder them.

Or we could try to find an IC that will help us out, but I've not had any luck finding anything.

My final suggestion would be to invent a Time Machine, go back in time and get Sega to fix this issue in the original MDs instead of finally getting round to it on the MDII.

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Re: MD hangs after playing any PCM file

Postby amushrow » Sat Apr 22, 2017 10:54 am

I'm going to make a board with diodes instead of the transceiver like I said, and see if it works out.
If the board turns out well I'll stick up a guide on making PCB's at home; with plated vias, solder mask, tinned traces / pads, and a durable coating for the cart pins. Unless it doesn't turn out that great, in which case I won't.

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Re: MD hangs after playing any PCM file

Postby prophet36 » Sat Apr 22, 2017 11:02 am

amushrow wrote:I'm going to make a board with diodes

Thank you so much for all your hard work so far on diagnostics. Thanks to you we now have a much better understanding of why some MD models suffer problems, and others don't. Also, thanks in advance for your work on this alternative bridge-board (irrespective of whether or not it yields fruit). This is exactly the kind of collaboration I hoped to see when I started the project.

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Re: MD hangs after playing any PCM file

Postby Jorge Nuno » Mon Apr 24, 2017 10:41 am

I just realized what it could be a conceptual issue. UMDK uses no CE_0.
So how does it know if the TMSS binary is executing or not? If it does not and forces the bus, irrelevant of the state of A14100 then it might be the issue that 2 devices are forcing data on the bus.

CE_0 is not active when the TMSS rom is on the bus, but CAS0 is always asserted for reads under 0x800000, irrelevant if it's TMSS or Cart.


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