==Background==
I am just starting out on the process of redesigning the MCU part of a battery powered datalogger. One feature it needs to have is an RTC or counter so that in the event of a low battery the device can enter some sort of low power state but that an external clock and counter remains powered.
The device requires a highly accurate clock with a low drift, unless I find a better alternative I will reuse the clock in the new design (as it's on a separate PCB and also very expensive!).
The clock has 3x outputs: PPS, 125Hz, 8.192MHz. The 8.192MHz is used to generate a 2.048MHz clock that is required by the ADC, The PPS is used for syncing the ADCs, measuring clock drift, and driving a counter in a CPLD which acts as the RTC in the old system (The main function of the CPLD is no longer required in the new design). The 125Hz output is not used.
Ideally I'll just use a microcontroller with a built in RTC and Backup power input pin, however it seems that they expect a 32.768kHz clock or crystal, the same is true of the dedicated RTC chips and 32bit counters that I've found. Most of these devices internall divide down to PPS signals anyway so it would be really useful to be able to just supply the PPS directly.
==Question==
Is there an easy way to get a 32.768kHz clock from a PPS, 125Hz, or 8.192MHz signal? This would open up the most options for device selection.
Or
Is there a (popular) 32bit MCU (cortex-m0+ probably), RTC IC or 32bit Counter that will accept one of the clock speeds mentioned? Ideally any counter or rtc will be able to read/set via i2c.
I think the Atmel samd chips have an adjustable rtc scaler which if I can set that to '1' will probably work. But I haven't been able to confirm yet.