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Motherboards Random question - how does CLRTC/CMOS reset jumper work?

Discussion in 'Hardware' started by tristanperry, 16 Aug 2020.

  1. tristanperry

    tristanperry Active Member

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    Hi all,

    Yesterday my system wasn't posting so after trying various things, I 'shorted' CLRTC on my Asus motherboard. As per various instructions, this means unplugging the PC and turning it off, holding a screwdriver to both jumper wires to 'short' it for 5-10 seconds, then plugging the PC back in.

    My question is - does this actually create a short? I mean, the PC is unplugged so no power is flowing (especially when you also remove the CMOS battery for this step)?

    As I say, a bit of a random question, it's just something that I was thinking since my system did seem to post fine after doing the CLRTC step.

    Thanks!
     
  2. David

    David Take my advice — I’m not using it.

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    The battery is providing the backup power for the BIOS. Removing the battery or shorting the jumper will clear it.
     
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  3. tristanperry

    tristanperry Active Member

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    Thanks for the reply, I tried removing the battery and my system still didn't post, but maybe that was coincidence or I didn't leave the system long enough on startup.

    The only thing that seemed to work is when I removed the power cable, shorted the jumper and turned things back on (as per ASUS' guide). But that's what confused me - if there's no power going to the motherboard, how does it 'know' that it's been shorted? It's not really important as my system's fine now, but I'm mainly curious.
     
  4. blackerthanblack

    blackerthanblack Active Member

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    I think it's always better to remove all power connectors from the PSU to the motherboard when resetting anything, just to be sure, even unplugged the PSU has some juice in it from the capacitors.

    For the reset of the CMOS, I guess you may not have had a good contact with the pins the first time, so there would still be some capacitance in the circuit giving power to the chip. For jumper pins the best connectors are the little bridge blocks that are made to fit on the pins, but I suppose these are rare nowadays.

    As to the shorting process - as David has hinted, the CMOS needs electrical power to keep any changed settings from the factory defaults. Stopping the power will clear the circuit and reset it to the factory default. Removing the battery for long enough will do this, and 'shorting' the pins is actually draining any energy that is in the circuit and capacitors. This can take a little time, which is why it's not instant - the chip won't be cleared until most of the power is drained.
     
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  5. Anfield

    Anfield Well-Known Member

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    Just smash the power button for the PC when the power supply is switched off, that'll drain them.
     
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  6. tristanperry

    tristanperry Active Member

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    TIL, thanks everyone :) Makes sense how the shorting process works even with everything unplugged etc.
     

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