> Modern motherboards use this "Standby" power to run small always-on
> power management circuitry (I believe this is integrated directly
> into the chipset these days) which monitors all sources of a "power
> on" signal, be it the power button on the case, or a LAN controller,
> or a built-in watchdog timer, or what-have you. When such a signal
> is received, this power management circuitry pulls pin #16 low to
> power up all the other voltage lines of the PSU, which fully turns
> on the computer.
That is essentially the same thing I was trying to say, though my wording might
have been confusing. My point was that it takes a special HARDWARE signal to
the power supply before it will start sending power to the main part of
motherboard (for the CPU, RAM, etc.). On older computers, this was just the
big red toggle switch. On newer computers, it is a "soft" switch.
On the newer ("ATX") power supplies there is is a small amount of power that
the PSU produces when the PSU is first plugged in which can be used to power
some small subset of circuitry (either on the motherboard or somewhere in the
computer chassis) that gets power whenever the PSU is plugged into the wall.
At a minimum, this will be tied to the "soft" power switch, but it may also go
to Ethernet NICs, clocks/timers/calendars, etc. But those devices must send a
HARDWARE signal back to the PSU to tell it to start sending power to the "main"
part of the motherboard (the CPU, RAM, etc.). Exactly what those devices are
and exactly what they do varies from computer to computer, and the settings
that control what they do are in the BIOS, not in ACPI. I use BIOS here in the
"generic" sense, covering standard BIOS, EFI/UEFI, etc. -- basically whatever
the computer needs to do before the "real" power is turned on.
The computer will automatically start booting as soon as the main part of the
motherboard gets power, and the PSU won't send power to the main part of the
motherboard until it gets a special hardware signal (the ground lead you
mentioned) that tells it to do that. The PSU doesn't care where the signal
comes from.
My overall point was that you can't turn on the PSU (to boot the computer) with
ACPI because ACPI only starts working after the computer (main part of the
motherboard) already has power and has booted an OS. You can't turn the PSU on
with software (like ACPI) that requires the PSU to already be completely turned
on, but you can turn the PSU off. While there is some overlap between what the
BIOS does and what ACPI does, they are two completely separate things.
> So yes, the motherboard does fire up some circuitry as soon as it
> gets power, but it doesn't boot per se.
Supplying a little bit of power to some peripheral circuitry, that may later
enable the computer to start booting by sending a signal that completely turns
on the power supply, is not booting up at all in my book.
> No, Bret, *you* are missing the point.
> This is not about the _motherboard_ getting power.
Yes, it is. The computer will not boot until that happens, and that won't
happen until the PSU gets an external signal from SOMEWHERE that tells it send
power to the main part of the motherboard. What "triggers" the boot process is
application of power to the main part of the motherboard. A modern PSU will
not do it automatically, but either the soft power switch or some hardware that
the BIOS controls can send the signal.
> This is a standard firmware setting on many PCs, both with BIOS and
> UEFI firmware. It is usually called “Restore on AC/Power Loss” or
> “AC Power Recovery”.
If it was "standard", then every computer would have it. None of mine do, but
it's good to know that it exists on some computers. And again, it's a BIOS
thing (BIOS in the "generic" sense) rather than an ACPI thing.
I think some of the confusion may have arisen when I was somewhat "loose" with
my interchanging the terms computer, power supply, and motherboard.
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