RTC battery

Just soldered a CR1220 battery retainer for the RTC which fits perfectly on the board:


The part number is BK-885 (bought from Digikey)

The module datasheet only mentions the purpose and maximum voltages on PMIC_BBAT. Is there any way to monitor the voltage?

@ShaneCCC

Please help him to set the voltage of backup battery.

Unfortunately I just fried my module but check in /sys/class/regulator and see if one of them has a name of “vdd-rtc”. It may tell you something.

There is nothing that links to vdd-rtc under /sys/class/regulator/.
Checking under the /sys/devices/c2a0000.rtc I can see some files which are showing the content of the rtc and get updated everysecond, but none of the data that resembles voltage of the rtc battery :(

found a max20024-pmic which has a regulator with a name that matches: vdd-rtc
under
/sys/devices/bpmp_i2c/i2c-4/4-003c/max20024-pmic/regulator/regulator.17
but the contents of the microvolts file shows 800000 which is only 800mV. This does not look like the one I’m looking for.

You can not read the charging voltage in SW. BBAT is supplying it. But
The PMIC’s builtin backup battery charging circuit can be configured using the CNFGBBC register of the PMIC.
The register exists at offset 4 for MAX77620 and MAX20024 parts.
Following configurations can be made:
Bit[0]: Enable backup battery charger
Bit[1:2]: Charging current setting
Bit[3:4]: Charging voltage limit setting
Bit[5]: Lower charging current enable
Bit[6:7]: Ouput resistor selection

Software configuration can be done using the MB1 PMIC cfg file. The file has sections for configuring PMIC registers.
Such sections have type=1 to indicate I2C transaction, i2c-controller-id=<> for instance of I2C controller and slave-add=0x78 for identifying PMIC.
[tegra194-mb1-bct-pmic-p3421-0000.cfg]
e.g,
T194:
pmic.system.block.type = 1; #I2C
pmic.system.block.controller-id = 4;
pmic.system.block.slave-add = 0x78; # 7BIt:0x3c

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May i know the purpose to monitor RTC backup battery voltage? Is your battery type rechargeable or non-rechargeable? The SW has enabled PMIC backup battery charging enabled by default, but HW prevents charging using Diode (D4 near J3). You must use non-rechargeable button cell battery CR1225 unless D4 is removed and zero ohm resister (R222) is mounted on pins close to D4. As Bbasu mentioned, there is no way to monitor back battery voltage. You can multimeter to measure battery voltage manually.

Thanks a lot @Bibek and @rkasirajan,
We are using primary CR1225 cells, which the devboard is not charging, luckily. I was not aware of charging functionality. Is there a document which I missed?
If the charging possibility is there, then I’d rather replace the cell with the rechargeable one and not worry about the battery level, after the board modifications you mentioned.

Attached RTC backup battery charging circuit & component location for your reference. Please remove D4 and mount R222 (0ohm) to enable backup battery charging.

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Hi All,

we now have the SMT Battery Clip and Rechargeable LIR1220 RTC Battery Kit on our webshop available now https://www.hazcam.io/collections/hazcam-kits/products/jetson-nano-rtc-battery-kit

regards, rapidproto