K22 RTC Clock Source

[laluquethibaut]

Hi,

For one of our application, we have to share a same reference clock for RTC module between multiple K22FN512VLH12  to keep the same reference time between all MCU.

According to datasheet and reference manual of the K22, the RTC can only take the XTAL32 oscillator as clock source. This oscillator is only defined as "analog oscillator" for 32k crystal and seems not to accept digital clock (no configuration available to switch working mode from crystal to digital oscillator). The problem is that a 32k crystal can't be share between multiple MCU...

My question is the following : Is that possible to replace the 32k crystal by a digital clock by connecting only the EXTAL32 to the digital clock ? And sharing it with multiple MCU ?

We made test on the FRDM-K22F board by replacing the 32k oscillator by a digital clock, it seems to work but we don't know if this result is repeatable and reliable. We won't take any risk hence my question !

in advance, thank you for your return.

regards

[egoodii · Accepted solution]

The K22 datasheets I am looking at have section 3.3.3.2 detailing an external-feed set of requirements, including Vec_extal32 for an external-amplitude of '0.7V to Vbat' (everything being on the SAME Vbat, I assume), plus associated notes 2 and 3.

If you can work with the uBGA package, I highly recommend the SiTime SiT1552AI-JE/Abracon ASTMTXK-32.768KHZ-LG.  I'm not sure if you would AC couple their 'NanoDrive', or just use LVCMOS hard-levels (depends some on distance).  These are VERY accurate (guaranteed <1s/day [11.6ppm] for E/G grade, plus a little aging), self-temp-corrected oscillators for 1.5uA.

[laluquethibaut]

Hi,

Thank for your reply.

Effectively, the answer is in notes of the electrical specifications of 32k oscillator.

We can supply the oscillator with an external digital clock on extal32 pin and left unconnected xtal32.

[egoodii]

Yes, as long as that 'external digital clock' drive is within the same Vbat-rail domain.