In-built RTC on Zero 3w

#1

The Zero 3W is my first Radxa. Rockchip, and non-RasPi SBC. So I was reading through the kernel boot log to try and understand everything available in the SBC.

While reading through the kernel log, I see the following 2 lines:
[ 15.495471] rk808-rtc rk808-rtc: registered as rtc0
[ 15.497696] rk808-rtc rk808-rtc: setting system clock to 2017-08-04T09:00:14 UTC (1501837214)

I am very pleasantly surprised to see a build-in RTC on the 3W board. However, I see that this isn’t very useful without some form of battery backup to keep the RTC circuit powered to keep time.

So my question is whether there is any way to attach some form of battery to the zero 3W to keep this time updated, instead of attaching an external RTC via I2C?

#2

Hi @samveen

We’re glad you’re using our products.

The kernel print log you read about the RTC is the RTC function that comes with the PMIC chip used by the 3W, it is not possible in hardware to power and keep the time up to date with the RTC circuitry embedded in the PMIC by means of an external battery.

#3

@Mitchell Thank you for that information. A follow up question on the PMIC RTC circuitry. Is this a read-only value or is it possible to write and updated value to it (e.g. using hwclock --systohc)?

I ask this because if this can be updated, then I don’t need to enable the fake-hwclock mechanism to keep the date monotonically increasing on this system, in the absence of an RTC.