Hacker News Reader

Comment by dale_glass | Hacker News Reader

> The ATX specification requires that the power-good signal ("PWR_OK") go high no sooner than 100 ms after the power rails have stabilized, and remain high for 16 ms after loss of AC power, and fall (to less than 0.4 V) at least 1 ms before the power rails fall out of specification (to 95% of their nominal value).

I don't think that quite works for the purpose. What you'd want is a second signal that goes low as soon as possible after loss of AC power.

My reading here is that PWR_OK going low is an indication that the PSU has stopped providing good power, and the CPU must shut down immediately, or it might miscompute something due to low voltage. At this point you absolutely don't want to do any last-minute writing, you'd be risking corruption.

What you need here is an early warning signal that you can react to while the PSU is still coasting on the internal capacitors.

View on HN

16ms is just longer than one AC cycle at 60Hz and less than one AC cycle at 50Hz.

I would has a guess that 16ms is the physical limit for most consumer hardware (and maybe commercial computing) to detect mains loss.

Of course there is industrial hardware that can detect quicker than this but it would add a LOT of cost for arguably little gain, or something that could be solved in another manner.

> I would has a guess that 16ms is the physical limit for most consumer hardware (and maybe commercial computing) to detect mains loss.

Doubtful. 16ms is an awfully long time these days. There's no reason why you couldn't detect power loss much sooner, given a good input signal. The concept also gets used quite often, in the form of SSRs with zero crossing detection. Those are used for dimmers.

The reason is likely related to the awful waveforms produced by some UPSes and inverters:

https://www.christidis.info/images/blog/scope_20.png

Unlike a nice sine wave, those spend a good while hovering near zero volts, so the PSU has to be able to tolerate that. Detecting loss of power sooner in this case isn't a question of cost, it's a question of that you don't have a good signal to do the detection on to start with.

> Unlike a nice sine wave, those spend a good while hovering near zero volts, so the PSU has to be able to tolerate that.

That wave chart was atrocious. I wonder if the extra load on the DC-side caps leads to them having lower life expectancy than the ones in a PSU attached to a proper power grid?

Power OK signals are used to prevent latch ups in silicon due to power glitches. The signals will route to power management ICs to ensure a full reset with proper bringing up of the power rails on any power glitch.

Reply on news.ycombinator.com