RTC Power Supply

[max00]

Hello,

as a friend of mine is just compiling kernel-modules for I2C-RTC support (some NXP-Chip, I think the same as used on the MOD-RTC UEXT module) I've seen a kernel driver for the A20-internal RTC.

After looking through the A20 Datasheet, User Manual and Eagle Files I've found, that there is an extra VDD-RTC pin available on the A20 Chip, which is connected to a 3V3 output of the power-management Chip on the board.

When I try to measure the Voltage of the Pin (board is running) I get only ~1.2V -> is it deactivated?
Also I was wondering if I could connect a 3V Lithium battery to this pin and provide power only to the RTC when the board is powered off - is this working that way?

Thank you,
Marco

[denix]

+1
Please some answer.

[essebie]

Good question, this can solve a lot of problems with date settings.

But the VDD_RTC signal is not available to a connector pin, it is on a track that arrives on a pair of capacitors, it could be difficult to connect the signal to an external battery.

[codifies]

I just noticed after pulling my LiPo that I lost the time (till now I didn't have ntpd installed)

I can only assume if you have a LiPo connected it keeps the RTC alive.... (You'll need to confirm this yourself with tests) it does rather seem the easier way to go!

[onefang]

I still don't understand why in this day and age we are still using batteries for RTCs, super caps would do a much better job.  Would be good if we could feed some sort of backup power into the RTC in the A20 without using a giant LiPo.

[ealex]

hello

I've been looking on the same problem.
In the schematic 'A20-OLINUXINO-MICRO_4GB_Rev_F1.pdf' VDD_RTC is attached to AXP209 (U12) pin 28 described as "LDO1 output，for Host RTC block"
Also, U12 has another pin called BACKUP "Backup battery pin" that can keep LDO1 up when all other power supplies are missing and can trickle change an attached battery / supercapacitor / etc when enabled from register REG 35H: Spare battery charging control.

I will try to get to pin 30, stick a super cap there and then change the AXP209 driver to configure register 0x35.

I'll post an update here when i gen to solder that cap - i need to get the board out and solder under a microscope.

Proposal for Olimex: on the next board revision leave a small pad / via for that pin. 

the info is taken from "http://linux-sunxi.org/AXP209#Pin_Description"