Pine A64 Plus Fails to Boot

[TinkerBear]

I sorta had my doubts that it could be the power supply, so I sat down to do some rigourous testing - three different power supplies to try, recording fluctuations on the power input (DC in and 5V lines) via Euler header and a DSO (Siglent SDS1052DL).

....

Wait - ONE MORE THING TO TRY - move that 10µF cap to the DC IN pin of the Euler connector...  And I'm booted and running off the PQI supply.  But not the 5V3A chinese supply.  Let's try that again.  Yep, works on 2nd try.  Hmmm, maybe bigger capacitor.

Yeah, me also, since I have a 8000mah lipo strapped to the underneath of the pine64, so it ain't gonna gonna get any ripple in the supply that way. It all seems rather strange... as this particular unit (which does have a WiFi module installed, but no ethernet) had been pretty stable prior to now, and then started having issues on reboots. However, I tell a lie (unless I had a different SD card inserted at the time - which is possible... I often swap that one between vanilla ubuntu and Armbian ubuntu depending on what it is being used for... it's a mobile test rig) it is currently on "ARMBIAN 5.25 stable Ubuntu 16.04.2 LTS 3.10.104-pine64", so if it was this image that was having issues, it is not 105 kernel related. However, I'm pretty sure it  exhibiting reboot roulette (i.e. have to keep rebooting it until it decides to work) until I plugged the HDMI in, but that could be something going on with the WiFi driver from what longsleep was saying.

If I didn't have a battery connected I would probably also try 1nf, 100µF (and a 1F supercap :-P) on the euler pins just to make sure things are smoothed out, but in my case it has a battery connected... so already has a bit one Or the PMIC just isn't doing it's job.

My somewhat vague knowledge of electrical engineering says that to fill a really fast dropout, you need something that can flow a lot of current in a hurry - and that typically means a ceramic capacitor.  Very low ESR and cheap... and not available in larger capacities.  Electrolytics can store a lot more energy, but can't give it up as fast.  (And supercaps are even slower, batteries slower yet.)  So I'm not sure if I need a bigger cap, or a ceramic.  And I have other things to work on today, so maybe tomorrow.