Power & Source of Big Ideas

M3 RTC battery pads on M3 board -- which is which?

Moderators: chensy, FATechsupport

Hi Folks,
The M3 has an on-board RTC. This loses contents when the board loses power -- no surprise.
There are two pads on the board that are for an external RTC battery; one pad is square and the other is round.
I have looked at the wiki, but can't find an answer. So ...
I have two questions:
i. Is the square pad shown as pin 1 on the schematic (i.e., +ve), with the round pad shown as pin 2 (i.e., DGND)?
ii. Is a CR2032 (i.e., 3 volts) sufficient to drive the RTC voltage regulator circuit when the board is unpowered?

Looking at the schematic, I'd guess so as the RTC regulator outputs 1.8 volts, which is what the RTC circuit needs.

[I also note that the M2/M3 schematic shows 2 GB of RAM (i.e., 2 x K4B8G1646Q, which are 8Gbit chips, whilst the board has 2 x 4 Gbit chips), so I'm not sure I trust it :]

all the best
Lawrence
(who would very much like to buy an M3 with 2GB RAM :)
Replying to my own mail -- sad.

Hi again Folks,
OK -- I didn't find the answer on the M3 wiki, BUT ... on the M2 wiki,
there's <http://wiki.friendlyarm.com/wiki/index.php/File:NanoPi-M2-1602-if.png>

Compare with <http://wiki.friendlyarm.com/wiki/index.php/File:NanoPi_M3_1512B_Dimensions.png>

SO ... ASSUMING that the M3 is laid out like the M2 -- **big assumption**, but reasonable as no answer from friendlyarm:
It looks like the square pad is indeed the RTC battery supply +ve (i.e., pin 1 on schematic) & round pad is DGND (pin 2).
I'm posting this just in case anyone else wants to keep time as well.

all the best
Lawrence
Another quick way to test this is to check each pad to GND on the GPIO connector. This will then let you know that the other pin is the RTC battery connection. Normally though the square pin is pin 1 but the GND check is worth doing so for confirmation.

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