BBB no power LED

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1

I know there are similar threads, but please remind me what the resolution to this is:

When I apply a clean 5V to the input jack, I often get no power LED (no LEDs at all). If I then press the power button, it comes on and boots fine.
The board booted fine for the first couple of months, and now it almost never does. Manually pressing the power button after turning it on is not acceptable.

Thanks,
Jim

2

Sounds like your power supply may be a little slow ramping up, may not be well regulated, or may be a little light on the current capability. No LED means the PMIC detected a fault condition and shut down.

You should not have to press the power button to power up. How does it work under USB power?

Gerald

3

I think it may be HDMI related. The problem started after I connected a monitor. I have yet to see the problem when the HDMI cable is unpluggeed. And then after it is unplugged and re-plugged, it works again for awhile. Very strange.
And I believe my power supply is OK - I’ve tried 2 different ones.

4

Check your grounds. Make sure the TV and the board have the same ground.

Gerald

5

A common ground doesn’t help, but I found another correlation. When I turn on the monitor first, the BBB comes on OK, otherwise it doesn’t. We suspect that the monitor in the off state is inadvertently trying trying to sink more current than the power over HDMI can source. What I don’t understand is, why don’t more HDMI source devices have this problem when attached to HDMI sink devices that are off?

6

That easy to check. It can only sink as much as the power supply can handle until the PTC heats up. Just measure the 5V at the PTC and see. The EDID circuit is supposed to be powered by the driving side, BBB, not the monitor and by law is limited to 50ma. What brand monitor do you have?

Gerald

7

Gerald,

I finally got back to this. The monitor is a 7" Lilliput 669gl. I checked SYS_5V as you suggested. It is 4.81V when the BBB is off, the monitor is off, and the monitor has DC power connected (so the monitor’s power supply is powering part of the BBB). If I then turn on the BBB, it doesn’t boot, and SYS_5V stays at 4.81V. If I then turn on the monitor, SYS_5V goes up to about 4.9V and it boots.

Case 2: Turn on monitor first. SYS_5V drops down to about 4V, then gradually climbs back up to 4.81V over a period of about 5 seconds. During that 5 seconds, the monitor has gone to standby (after reporting no signal). Turn on the BBB and it does not boot.

Case 3: Turn on monitor first, power up BBB during the short window when the monitor is displaying “no signal” and SYS_5V is climbing from 4V. In this case SYS_5V jumps up to 4.9V, the BBB boots, and everything works great.

Case 4: Disconnect DC power from monitor - boots fine.

Case 5: Disconnect HDMI cable - boots fine.

Thanks,
Jim

8

Sound like the monitor is out putting 5V on the HDMI connected. Not good. Violates the specification.

Try removing RT1 from the board and see if that helps.

Gerald