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  • Joined: 11 Aug 2007
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  • Location: Coimbra, Portugal
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Purpose of Game Gear's 1.28 Vref
Post Posted: Thu Oct 08, 2020 11:26 am
Hey guys!

Long time no see, glad to see this forum is still alive :D

Tired of dying game gear's power supplies I've replaced them with a couple of step-up and step-down converters to get the required 5V and 34V voltages required by the system.

However, it seems that the 1.28V Vref voltage previously generated by the MB3775 is also important. I say this because many times the game gear does not boot the ROM unless this voltage is present. I say "it seems" because there are also times when the GG indeed boots. What I am sure is that the GG always boots fine with a 1.28V Vref present. So it seems it is required.

My question is, does anyone know the true purpose of this 1.28V Vref? I'm unable to understand its purpose just by looking at the GG's schematics. I would need to go inside the ASIC(s).

Best,
CK
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  • Joined: 28 Sep 1999
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Post Posted: Sat Oct 10, 2020 3:26 am
C3R14L.K1L4 wrote
I say "it seems" because there are also times when the GG indeed boots


For these times when it boots without the 1.28V on Vref I assume the audio and video are OK and there don't seem to be any other problems? Does the power LED stay lit?

For the times it doesn't boot, is the power LED on as well?

If you disconnect the MB3775 Vref and just have a voltage divider off the 5V supply to produce 1.28V does that work?

In the VA0 schematics the chip that handles I/O, the LCD, and resetting the system gets Vref. It doesn't seem useful for I/O, so I wonder if it's used to detect some kind of low power or power failure condition so it can keep the rest of the system (CPU, VDP, maybe LCD control signals) in a safe state as long as possible until the power is gone.

If that situation was detected abnormally due to Vref floating it could look like a failure to boot, when really it's "I think I'm about to shut down, better keep everything reset". That might also explain the randomness too.
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  • Joined: 05 Nov 2014
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  • Location: Auckland - NZ
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Post Posted: Sun Oct 11, 2020 4:20 am
Its probably used as a reference for an analog to digital converter. My guess would be to be able to measure battery level as from memory there was a direct battery connection that goes to the main board. As mentioned above.. it may also be used as part of a power up/power down/reset circuit, probably determined by the result of the adc which would indicate battery level. Seeing what the power led does when it wont boot may help to prove that.
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  • Joined: 11 Aug 2007
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  • Location: Coimbra, Portugal
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Post Posted: Mon Oct 12, 2020 12:53 am
wasup wrote
Its probably used as a reference for an analog to digital converter. My guess would be to be able to measure battery level as from memory there was a direct battery connection that goes to the main board.

According to schematics that may be done using the voltage divider and ASIC's Vres/Vonf pins (follow the power supply connector in the mainboard).

Charles MacDonald wrote
For the times it doesn't boot, is the power LED on as well?

If you disconnect the MB3775 Vref and just have a voltage divider off the 5V supply to produce 1.28V does that work?

If Vref is left floating the system is unstable as I described before. If Vref is set to 1.28V reference (I am using an TO92 LM317 for that) the system works perfectly. When Vref is grounded most of the times the system boots fine and others it hangs immediately with garbage on the screen (see attached image, the game is Columns).

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Post Posted: Wed Oct 14, 2020 9:48 am
Just to add that the GG consumes a constant 20uA from Vref.
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