[SAME70] RTC backup with battery

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Hi,

 

I'm Trying to get the RTC time saved when my board is no more powered. So I added a lithium battery on the VDDIO pins with diodes so that VDDIO is powered by my 3v3 regulator when the board is powered, and by the 3v lithium battery when it's not.

This works well, and time is preserved.

 

The problem is that there is way too much current used by VDDIO. I measured 7.8mA at the output of the battery. I checked my design, and the VBat only goes to a BAT54C diode, then to my VDDIO pins. Nothing more.

 

I also see something strange: on the Vout pin, there is about 250mV instead of nothing, making me think the internal voltage regulator is not really switched off. I tried enabling the brownout detector and brownout reset, but that didn't help. What I expect is the MCU to consume only a few µA as stated in the datasheet, when it is in backup mode. Maybe I missed something and it doesn't go to backup mode?

Last Edited: Mon. Jun 13, 2016 - 06:55 AM
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Hi, I'm trying to solve similiar problem on SAM4S2C. When I go to deep sleep on completely powered board and measure current to VDDIO then it is about 15uA. But when I turn off power source of board and MCU runs only from batery on VDDIO (in deep sleep), then current to VDIO is cca 0,7mA. It looks llike there must be some current leakage through PIO pins. I was triying many combinations of pullup and pulldown disable/enable and PIO disable, but nothing helped. Anyway there is zero volts on VDDOUT.

 

Milan B.

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I can't use my RTC in this case. I can't ask user of the board to cleanly shut it down before unpower it. Did you have more success since your post?

 

Just for curiosity, what is your code to go in deep sleep on powered board? And what happen if you unplug it after that?