Pulsing Power Up

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1

Hello,

I just finished reading a lot of posts on the topic of starting up the BBB and various issues that folks have had. I didn’t see anything that reflected my situation, so I’m going to reach out in hopes that someone in the group can cast some light on this.

I have a BBB with a custom Cape. The Cape is designed to interface the BBB to a cap-touch color screen. The Cape contains the backlight power supply for the display and the obligatory EEPROM for ID, addressing pins, etc. Very similar to the BB-CAPE-DISP-CT43 available for purchase.

What is different in my case is that I’m supplying power to the Cape (5V). The power then goes to the Cape connector on P9, pins 5 & 6. This would be equivalent to supplying power to the BBB via the barrel connector.

The BBB is internally booting (no SD, etc) with custom code. I have written the shipping code image into the board and I get the same results.

I’ve made several dozen of these assemblies now, and so far I have two that exhibit this odd behavior.

The symptom: The BBB will attempt to start, will shut down, and will try to start once more at a rate of about once per second. Sometimes this only happens a couple of times, other times I simply give up after 60 of these failed boot cycles. There is no consistency to the number of tries a board will go through.

I’m using a very robust power supply that meets or exceeds the PMIC slew rate requirements. The supply is capable of supplying 12A @ 5VDC.

When the BBB is attempting to boot, the blue LED adjacent to the barrel connector (commonly referred to as the Power LED) will flash briefly.

There are occasions when a ‘bad’ assembly will boot without issue, but most frequently it comes up in this flashing state.

I’ve checked for the usual suspects, bad solder joints, interconnects, etc, without finding anything. I know that my inrush current is on the order of 1.7 to 1.8 A at the initial startup, but that drops off significantly after the caps are charged up.

Has anyone out there experienced this pulsing behavior and found a root cause and possibly a fix?

One thought that I had was to monitor the bebug port (X1), but is there even enough up time to have this be useful?

Thank you to everyone in advance for your insight.

2

Still sounds like a power problem.

A lot of 12 Amp supplies might go out of regulation at almost no load.
They typically need to be loaded to ten percent of max rated output load to be within Voltage spec.

The Beagle supply needs to stay between 4.5 and 5.5 Volts under all load conditions.

I recommend you stay between 4.75 and 5.25 to have noise margin against the PMIC going into self-protect.

— Graham

3

I have a static 1.5A resistive load on the supply which has a 1A minimum load. That would leave me with 10.5 A of wiggle room. Out of which I need just less than 2A.

I’ll revisit the supply level, but I’m fairly certain that I am well within those ranges.

4

I’l look at it with both a Voltmeter and an oscilloscope.

I have a B&K bench supply that when you turn it on, with a BBB already attached, did an overshoot up to around 5.7 Volts before settling back to 5.0 Volts.
It would trip the self protect on the PMIC almost every time you tried to start the BBB.

— Graham