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   Chris Siebenmann (cks@mastodon.social)'s status on Monday, 09-Sep-2024 12:40:53 JST Chris Siebenmann

   Today's interest: just how does a modern PC motherboard implement soft ATX power control? Presumably the main CPU isn't running all the time, although parts of the motherboard are powered. Is there a separate little always-on SOC that implements the logic? Something more clever?

   The information is probably somewhere on the Internet but my search luck on this is 'lol no', probably since I don't know the right technical terms to look for.

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     Haelwenn /элвэн/ :triskell: (lanodan@queer.hacktivis.me)'s status on Monday, 09-Sep-2024 12:40:53 JST Haelwenn /элвэн/ :triskell:

     in reply to
     @cks As in suspend-on-ram / S3 deep-sleep?
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     Chris Siebenmann (cks@mastodon.social)'s status on Monday, 09-Sep-2024 13:46:48 JST Chris Siebenmann

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     • Haelwenn /элвэн/ :triskell:

     @lanodan I was thinking more things like how the BIOS setting for 'when power is restored, stay off/turn on/return to last state' is actually implemented. But apparently that is probably entwined with the S3/etc sleep logic, which I guess I shouldn't be surprised by.

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     Haelwenn /элвэн/ :triskell: (lanodan@queer.hacktivis.me)'s status on Monday, 09-Sep-2024 13:46:48 JST Haelwenn /элвэн/ :triskell:

     in reply to
     @cks deep-sleep logic at least somewhat makes sense to me, I think it's mostly turning of the peripherals and a special mode on the CPU similar to turning cores off.
     While power back to last state is weird to me given how I conceptualize ATX power as "pushing the power button pulls ATX pin 16 (green) low and then mobo keeps low while on".
     Kind of makes me wish someone like Ben Eater would explain it. ^^
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     Haelwenn /элвэн/ :triskell: (lanodan@queer.hacktivis.me)'s status on Monday, 09-Sep-2024 13:59:53 JST Haelwenn /элвэн/ :triskell:

     in reply to

     • Haelwenn /элвэн/ :triskell:
     @cks Although maybe pin-8 (grey) gives the PSU state even without being truly on so you could then use that and a simple one-bit or parallel chip that the bios dynamically updates when it's set to restore power and otherwise leaves it static when it's set to either leave-off or always power-up on AC-back.
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     Haelwenn /элвэн/ :triskell: (lanodan@queer.hacktivis.me)'s status on Monday, 09-Sep-2024 14:43:21 JST Haelwenn /элвэн/ :triskell:

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     • Haelwenn /элвэн/ :triskell:
     @cks Okay tested two of my spare PSUs (Antec from 2008, Huntkey of probably the same era) with a voltmeter.
     With just AC attached pin-8 (grey) is neither pulled up or low, but as I then noticed in the documentation pin-16 (green) gives a 5V signal and it can be measured without fully turning the PSU on (they're old PSUs where the fan turns on whenever pin-16 is pulled low).
     So I guess pin-16 is used as "check AC-back setting" signal.
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     Erin 💽✨ (erincandescent@akko.erincandescent.net)'s status on Monday, 09-Sep-2024 16:22:37 JST Erin 💽✨

     in reply to

     • Haelwenn /элвэн/ :triskell:
     @lanodan @cks CPU is off. Some bit of chipset/BMC is powered by +5V Standby, often the embedded controller or management controller
     Haelwenn /элвэн/ :triskell: likes this.
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     Chris Siebenmann (cks@mastodon.social)'s status on Tuesday, 10-Sep-2024 11:09:13 JST Chris Siebenmann

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     • Haelwenn /элвэн/ :triskell:

     @lanodan Are PC power switches wired directly to the PSU? I thought they all went through the motherboard these days, which is how things like 'hold for four seconds to really power off' and 'just pressing briefly sends a signal to the OS' worked. That means there's something on the motherboard that interprets them, even when the system is nominally powered off. (And then pulls PSU pins to actually deliver (more) power.)

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     Chris Siebenmann (cks@mastodon.social)'s status on Tuesday, 10-Sep-2024 11:09:13 JST Chris Siebenmann

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     • Haelwenn /элвэн/ :triskell:

     @lanodan According to what I looked up, pin-8 (grey) is only valid when the ATX PSU is officially on; it's a signal that what the PSU is supplying to the system is good and you can start using it. Pin 9 (purple) is +5V standby power and is probably the primary 'do we have AC' signal that things use (well, or power themselves from it). On desktops, all the power management (including 'power up or not after AC on') seems to be in the ICH/PCH chipset stuff, or the equivalent for AMD.

     Haelwenn /элвэн/ :triskell: likes this.