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Beginner
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Intel NUC 8 i7 HNK-NUC8i7HVK IO COmmon Header PWR_LED_P stays 5V even after computer has shut down

I am using my NUC inside a mobile cart so I have routed an external power button to turn the PC on and off. I've connected to the IO Common Header (found on page 42 of the TechProdSpec) using the GND and PWR_BTN_N (pins 14 & 20) to turn the PC on. This works as intended.

 

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The button I'm using also has a LED ring on it to show if the PC is on or off. I've connected the leads on the button's LED ring to PWR_LED_P and PWR_LED_N (pins 10 & 12). This works most of the time, turning on the NUC the light comes on and shutting down from the windows menu it turns off.

 

The issue I'm having though is when try to turn off the machine by holding down the power button to, whether my external one or the one built into the NUC, the LED ring stays on indefinitely. It will not turn off unless I unplug the NUC's power cable or if I press the button again to turn on the computer it briefly turns off then comes back on once the computer has posted.

 

Anyone have any idea why the PWR_LED_P is staying at 5v even after the PC has shut off in that one scenario?

 

 

 

 

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Super User Retired Employee
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This is normally used to light up the LED in the Power Button when the system is off. I believe that you have control over this from the BIOS and from the LED application for HN/HV.

...S

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Beginner
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Hi Scott,

 

Had to move to a different project for a couple days and just got around to looking through though the LED controls in the bios. I see options for controlling all of the LED's on the front of the unit, but nothing for the common IO header. I'm a bit confused by what you said about that light normally being used to turn the LED on when the system is off. I know some computers will blink an LED if its asleep, but I'd think the light being on would mean the system is on not off. Can you clarify on that? I am going to try to do a bios update to see if that fixes the behavior I'm seeing, but I believe I'm currently on the latest version. Or would it be better to just power the LED off the VCC5 pin, as I'd assume that would turn off when the computer is turned off which is my desired functionality.

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Super User Retired Employee
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Some of the NUCs (obviously not all; read on) have an ability to light the power button at a dimmer level when powered off (perhaps so you to find your way to it in the dark?). Now that I think of it, this may not be the case with HN/HV as their power button does not light when powered off.

 

I will also mentioned that I have had a number of occasions where I had to manually enter the GUI and reload my configuration from file before the LEDs, etc. would act the way I wanted them to. This tells me that there might be a problem in the Embedded Controller that implements this LED management.

 

A thought: try configuring USB to be off in S4 and S5. Maybe this 5V is coming from the same circuit as the USB power for these states?

 

...S

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Beginner
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Hmmm, so I just looked and "USB s4/s5 power" was already unchecked. I did also turn off portable device charging mode but that didn't seem to help. I agree it seems very likely that USB is staying on for some reason causing that pin to have 5V's on it even when it shouldn't. I did realize I was out of date with the bios (upgraded from 0049 to 0059) but no luck there either. I disabled software control for the LED's in the bios and then disabled each LED but again my external power button still stays lit. I am curious if you think there would be any consequences using VCC5 and GND instead of PWR_LED_N and PWR_LED_P. I assume VCC5 would turn off with the PC as theres a separate 5V_STBY pin for if the computer is powered down or asleep?

 

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I did also notice, the LED only stays on if the computer has booted to windows and the power button is held down. If you're in the bios and press or hold the button down, the LED immediately turns off like it should. No idea why its behavior would change once it boots into windows. So in every case but 1 the PWR_LED pins work correctly, its only when booted into windows and the button is held down that it fails to turn off.

 

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Super User Retired Employee
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When you are in BIOS Setup and press the power button, this is being accepted and responded to immediately. When you hold down the power button for 4 full seconds while in Windows, on the other hand, the PCH is pulling the rug out from underneath everything. In this case, the BIOS never gets an opportunity to disable the circuitry that allows power to be conditionally routed to the various circuits for USB S5 Power, etc. As a result, these circuits stay powered.

...S

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Beginner
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Huh, interesting that does make sense. So theres really nothing that can be done for this it sounds. I'm okay switching to a non-lit power button, it just would have been nice to have it but it would end up being confusing for our users if the light were to stay on when the machine is off. It is weird that the actual power button on the machine turns off in that scenario, but i guess those must be on separate circuits for power. Just out of curiosity do you know how other computers handle this as most desktop cases have LED rings on the power button and they turn off when holding down the power button from windows and whats different with the NUC?

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Super User Retired Employee
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Ok, somewhat facetious response, but everyone using standard NUCs will be seeing the same result. That said, shutting down by holding the power button for 4 seconds is NOT normal and should NEVER be done if it can be avoided (you CAN corrupt your drives this way). If a normal shutdown of Windows is done, the power should be turned off.

...S

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