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Weird EPROM issue with Sega Saturn Region Free Bios

ziggaboogi

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Hello,

Having a very strange issue with a VA0.8 US Sega Saturn that I've been trying to put a region free BIOS into.

I'm using AM27C400-150DC chips but have found that for some inexplicable reason, the system will only boot if there is light hitting the window of the BIOS EPROM. This means that if I put anything over the window (piece of electrical tape, the rf shield etc.) it will not boot correctly. However if the window is not covered and is being hit by light then it boots up just fine and I was able to play my copies of Nights (JP) and Daytona USA (US).

At first I thought it was a weird chip defect but even after ordering several more I am still having this exact same issue. A region free BIOS is kind of useless if I can't actually close the system back up.

Does anyone have any idea what's wrong? Do I just need to source a completely different chip because by some freak accident I found the only condition that this model works in?

Edit:

I did verify all 40 pin point connections and the system has zero issues when I reinsert the original BIOS chip
 

Trimesh

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Try using faster EPROMS - say 100 or 120ns. I've never seen this problem on a Saturn, but I have seen EPROMs that had faster access times when the chip was exposed to light, which could well cause the problems you are seeing.
 

ziggaboogi

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I did consider that and it looks like the original bios chip was around 10mhz. So I did source the 105ns version of the same chip so hopefully that extra bit of speed is the solution and I just by some fluke found the one way to make the 150ns version of it boot.
 

Druid II

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For what it's worth, Sega themselves used 27C4100DC-12 chips in the Saturn.
 

Trimesh

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I did consider that and it looks like the original bios chip was around 10mhz. So I did source the 105ns version of the same chip so hopefully that extra bit of speed is the solution and I just by some fluke found the one way to make the 150ns version of it boot.

Threshold shifts on EPROMs when they are exposed to light is something that's known to happen. Years ago I was working on a design that suffered from a strange "sometimes crashes when completely assembled" problem, and that turned out to be the same thing. You can actually demonstrate it quite easily on a breadboard - drive one of the address lines with a function generator then display both the incoming address line and the data line on a scope - the extent varies from device to device, but you can consistently see Tavqv dropping proportional to the incident light on the die with most vendors.

Another trap for the unwary is that for a lot of vendors the stated speed grade is effectively cosmetic - you can get a vendor that's selling "-100", "-120" and "-150" parts and in reality they all meet the spec for the -100 part with the slower grades effectively only existing so that people that were using the older slower parts don't need to change their ordering part numbers. This can bite you because it means you can get through characterization testing using a part that technically shouldn't work but does - but then suffer from failures if you ever change vendor to someone that is selling parts that really do operate at their marked speeds.
 

ziggaboogi

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Thanks for some more actual EE background to this issue. In a lot of other places I've posted this issue they kept telling me that I was wrong and clearly something else in the system or process must have messed up (which I'm still not ruling out 100%). Or that clearly it's because I was adding pressure or something when removing the tape.

So I did source two 105ns chips and they are still exhibiting the exact same symptoms with the light.

Would it be worth trying the 27C4100DC-12 chips as suggested by Druid? I've already spent more than I intended on Eproms for this project.

I'm looking at the spec sheet for both the 27c4100 and the AM27C400-105 and the pinouts and access times should be compatible. Is there anything on the system itself that could be causing this issue?

I feel like the odds that 8+ chips of different speeds having the same issues is getting harder to call a chip issue. Though I have been getting them all from the same vendor which could also explain it.
 

Druid II

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You could also consider the possibility that you are being sold chink counterfeits or damaged stock.
 

ziggaboogi

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While I don't appreciate that term being used as I have family in China. I do acknowledge that I could totally be getting damaged/fake chips from some podunk Chinese factory or vendor.

For now I've just wired up an led as it's not worth pursuing a refund for the amount of money and I'm not willing to sink more effort for a personal unit. If this was for someone else I
wouldn't go with such a jank solution but, if it works it works I guess.
received_367033630952500.jpeg
 

Trimesh

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At this point I also tend to suspect some sort of problem with the parts - I've never found the Saturn boot ROM to be even slightly critical and I've used various speeds (-100, -120) for it with no problems. It could conceivably be a problem with the programmer (like VPP too low) that affects the programmed levels, but that also seems doubtful.

I have to say that's a creative approach to solving the problem :)
 

Nully

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If you're in the USA I'll send you one for free.
 

ziggaboogi

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So an update. Nully was kind enough to send me an EPROM preburned with the RF BIOS. Unfortunately the problem still persists which tells me that this has to be some kind of fault in Saturn itself, or perhaps a very strange quirk of this revision. For what it's worth it's a US VA0.8 but again at this point I have a working region free Saturn (which also just got a phantom chip ;) ) so I'm not too keen on tearing into it again to find the root cause. Hopefully this thread will help any one who runs into a similar issue.
 

Nully

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I have no idea, didn't think it was your chip to begin with though. I don't understand the LED thing and would instead just retouch your joints, maybe try a machined socket instead of those $0.02 ones. Make sure all the pins are in and pushed all the way in etc. Make sure no pads have accidentally been lifted or damaged. I've probably installed those in 100 Saturns and never had a problem. If you really want, I can see if I have a spare board if your Saturn is actually cosmetically-worth fixing.
 

ziggaboogi

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At this point I've reverified the continuity on all 40 points twice and did touch up all the joints again as a just in case. Though I am using one of the regular sockets and not a machined one. Thank you for the offer of the board, but this Saturn has always been a little cosmetically beat up to begin with so that's probably better used for someone with a better condition shell or a completely dead Saturn.
 

gorgyrip

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I'm using 27c160 on va 0.8 without a problem.
 
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