SOPine modules stay off at reboot?

[Pine]

Adding an SMT Schottky diode (the one already in the SOPine schematic, XBS104S14, would be fine) would fix the above-described issue, which is actually rather easily doable due to the Clusterboard PCB layout.  If you follow the pad to which the plus pin of the battery holder is soldered, there is a very long trace on the top side of the PCB that goes along almost the entire left-hand side of the PCB.  Cutting that trace and soldering in an XBS104S14 diode is all that's needed to prevent the AA batteries from becoming charged.

That is clever. This is the line you are talking about. An SMD Schottky diode would work. With a microscope, an Exacto knife, and a bit of patience, it will work. One issue I had with soldering is that the whole top layer is a heat conductor, so I had to tin my soldering iron and apply flux to the board and only transfer solder from the tip to the SMD, not heating up the pad first. About one second of touching the soldering iron should do it.



Another idea I was toying with is to uncrimp the +RTC terminal in the holder with a fine jeweler's screwdriver or Exacto knife, pop out the tiny center piece of metal (the battery terminals are rigid loops), and interpose a diode with a new + contact for the holder.


> An external 3.3 V source could be used, for which an input is already conveniently routed from the Clusterboard's 24-pin ATX connector to one of the R351 pads..

Exactly. What I'm actually going to do personally is, I'm going to use the Molex connector where I already supply 5V and add a 3.3V line on Molex pin 2 with barrier diodes to batteries. R351 (placeholder) runs near the +RTC, so another barrier diode will fit nicely there with a jump wire as you described. 


> With the additional SMT Schottky diode in place, two AA batteries should provide about 2.7-2.8 V (when new) to the VCC-RTC pins of the A64 SoCs on SOPine modules, but the actual RTC backup functionality (as concluded in your research, and confirmed in the A64 datasheet) should work just fine with as low as 1.8 V.  In other words, batteries would work fine (and not just while they're brand new) as the RTC backup (i.e. while the Clusterboard is powered off) with the additional SMT Schottky diode on the Clusterboard.

Min 1.8V supplying 0.45mA to all 7 SOPines while powered off according to the A64 specs will do it.


> Also, the RTL8370N switch ASIC should take around 2.8 W or 0.9 A from the U1, which leaves us a lot of headroom until reaching the U1's maximum output current of 2 A.

I popped in 7 SOPines and ran heavy networking and the switch didn't suffer. When powered, the 7 SOPines draw 3mA from RTC. Lots of headroom left.


> However, I wouldn't take 3.3 V from the cathode of D1; instead, I'd simply remove a small part of the solder mask from the thick PCB trace that the pin #3 of the U1 is soldered to...

That will work. You could even solder a thin wire on L1. I only chose D1 because it is after the LC filter (the U1 voltage out waveform is noisy so a big choke is needed), there is a bank of capacitors available after that L1 choke (power cycle but keep RTC powered for a few seconds), and D1 is closest to the +RTC terminal.

I'd personally remove the holder. A super-easy way is to uncrimp the terminals as described before, lift the holder straight up and out, and then clip the terminals or put the holder back if you add another diode. You don't even need to unsolder the battery holder. Pretty neat.