VOGONS

M919 - single DPDT switch for dual FSB/CPU voltage selection possible?

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First post, by bertrammatrix

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Poking around my spare m919 and trying to figure out how to easily incorporate a switch from 60 (180) mhz and 3.6 volts to the cpu to 66 (200) mhz and 3.85 volts without opening the case and messing with jumpers every time, I think I had an epiphany....or a brain fart (sometimes hard to tell). I quickly drew up a schematic, hoping to get some input before trying to test it.

The attachment 20251118_122537.jpg is no longer available

Now the JP4/cpu voltage portion on the right will obviously work just fine like this, no question there

However the JP3 frequency selection portion I'd like to confirm. The pins from this jumper block are connected to ground on one side, and it looks like directly to the neighbouring UM9515 ic on the other, for which I am not finding any datasheet (no surprise). In the normal/open condition they are all just floating until you strap them to ground. This being said - would it bother anything if I tie pins A and C together, since in the two selection scenarios I need they either BOTH need to float, or they BOTH need to be grounded anyway? My thoughts are that it should work, however I'm not sure if I'm missing anything here

I'm sure there's more moderm and elegant ways to do this, however I have a pushbutton DPDT switch here and I like to K.I.S.S. whenever possible

Reply 1 of 3, by RetroPCCupboard

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I am no expert, so looking forward to more qualified replies. But, I was intending to do similar on my Super Socket build.

My thoughts are that when the switch is closed then both connected to ground (probably through a resistor), which is exactly the same as using the jumpers. The difference is when it is open.

In a normal scenario both pins are separately floating. If you connect them with a switch then they are connected together whilst floating. Without a datasheet it's hard to know, but if there are pull-up resistors inside then tying the pins together will put those resistors in parallel, and therefore half the resistance. Possibly increasing current. I am not sure if that's a problem TBH.

Reply 2 of 3, by bertrammatrix

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RetroPCCupboard wrote on 2025-11-19, 08:35:

I am no expert, so looking forward to more qualified replies. But, I was intending to do similar on my Super Socket build.

My thoughts are that when the switch is closed then both connected to ground (probably through a resistor), which is exactly the same as using the jumpers. The difference is when it is open.

In a normal scenario both pins are separately floating. If you connect them with a switch then they are connected together whilst floating. Without a datasheet it's hard to know, but if there are pull-up resistors inside then tying the pins together will put those resistors in parallel, and therefore half the resistance. Possibly increasing current. I am not sure if that's a problem TBH.

Right, thanks, I think that helped ansewer my question. Poking around in the vicinity more I am not able to find those 3 lines connected to any pull up resistors, or anything else other than the IC and the selection pins. Additionally there is zero continuity between the 3, which wouldn't be possible if they were all strapped to + via pull up resistors

I think I'm in the clear to try this out when I get some time to wire it up

Reply 3 of 3, by RetroPCCupboard

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Ok. Good luck. Please post back with results

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