04-20-2021, 01:20 PM
(This post was last modified: 04-20-2021, 01:22 PM by dsimic.
Edit Reason: Additional clarification
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(04-14-2021, 03:30 PM)Pine Wrote: Since I found the diode, I've added about 20% more to that page I made. I tried to model the schematic as well. One discovery I made is that PWROK drops to 2.25V without the RTC battery, but stays at 3.25V (+0.25V due to voltage from the 74LVT04) with an RTC battery. In the baseboard, PWROK stays at 3.00V, of course (no 74LVT04).
I went through the current state of your web page, of course, and your latest discoveries are great! It is really satisfying to see both hardware (i.e. manual) and software resets working, after putting so much time and hard work into the analysis and debugging. Once again, an awesome job!
(04-14-2021, 03:30 PM)Pine Wrote: About the RTC battery fighting a losing battle, it looks to power all 7 SOPines, yes, then gets weaker, and then gets overpowered by the combined VCC_RTC output of the 7 SOPines. The baseboard solves this with diode OD3. I'll opt to not use batteries going forward.
Exactly, the two AA batteries are connected directly to the VCC-RTC pins of the A64 SoCs on the SOPine modules on one side, and to the VCC_RTC outputs of the AXP803 PMICs on the SOPine modules on the other side, so nothing prevents the combined VCC_RTC outputs of the seven PMICs to start charging the AA batteries once they get weak, which is certainly very bad. This power path should be always disabled, regardless of any further modifications, if you agree.
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.
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.
My desired outcome is to have the resets working properly, and to have two AA batteries in the existing battery holder as the RTC backup, without the batteries becoming discharged (or, even worse, charged) while the Clusterboard is powered on. As a note, adding the above-described SMT Schottky diode would probably prevent the batteries from being a fix for the reset issue, but the diode must be added anyway. Based on your latest research and my own further investigation of the SOPine and Clusterboard PCBs and schematics, there are two ways for accomplishing the desired outcome, and I'll describe them below.
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 (more precisely, the right-hand one). As we know, R351 is the large resistor that can be optionally soldered to the PCB to provide a dummy load for the connected ATX power supply, and is usually not needed. It is rather easy to solder a thin wire that connects the right-hand pad of the R351 and any point in the above-described long trace that's after the additional SMT Schottky diode. Of course, another through-hole Schottky diode needs to be placed "within" that jumper wire, which is also rather easy to do with some heatshrink tubing, and nearly everything can be tucked away rather neatly on the bottom side of the Clusterboard PCB.
The above-described way is particularly handy when using an ATX power supply to power the Clusterboard. It can also be used with a completely separate 3.3 V power supply, or with a 3.3 V "tap" (i.e. a separate step-down converter) going from an already existing 5 V power supply that powers one or a few Clusterboards.
As you've already described it on your web page, another way is to tap into an already existing, internally regulated 3.3 V power source on the Clusterboard. As you've already pointed out, DVDDIO can be used as a 3.3 V source; as a note, the U1 (RT8284) is, according to the RT8284 datasheet, configured to give 0.923 * (1 + R3 / R7) = 0.923 * (1 + 26.1 / 10) = 3.332 V at its output. 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.
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, and solder a thin wire to it. The wire can be threaded through one of the pin holes of the unpopulated J33 ("Molex") connector, routed underneath the PCB, and threaded through the left-hand pin hole of the unpopulated R373, to bring it again to the top of the PCB. The other end of the wire needs to be soldered to any point in the above-described long trace that's after the above-described additional SMT Schottky diode. Another through-hole Schottky diode needs to be placed "within" that jumper wire, as already described above. Also, this approach doesn't require the battery holder to be removed, which is a plus.
I would love to hear your comments.