Take advantage of external RTC to avoid 32.768KHz duplicity?

[jemajuca]

Hello. I decided to use an external solution for RTC. Sometimes the RTC chip offers an buffered 32.768KHz output. Can I use this output to supply the 32.768KHz to the nRF52832 and gain a litle power saving? Thanks in advance.

[Sigurd]

Hi,

In general, it’s no problem using an external 32.768 kHz clock source. But, if the external 32.768 kHz clock is only sometimes available, you would need to switch between the RC oscillator clock and the external clock.

Unfortunately, you cannot change the SoftDevice LF clock source on the fly, since this would disrupt the protocol timing. You would therefore have to disable the SoftDevice, and enable it again with the new clock source. If you are in an active connection, you will be disconnected, and therefore have to re-connect every time you change the clock source. In most applications this would not be acceptable. The potential power saving is also very small, with the RC oscillator only using about 0.6 µA.

[jemajuca]

Hello Sigurd. The external RTC chip would be powered by the same power supply than the nRF, its seems not a problem to lose oscillation except at startup than can be 1s. I think the power saving can be considerable because the external RTC only needs about 200nA. Can the long startup be a problem? For the connection, I guess input is pin2 P0.00/XL1. Is correct? Thank you.

[Sigurd]

P0.00/XL1 is correct. What do you mean by "its seems not a problem to lose oscillation except at startup than can be 1s ?" ? It takes 1s to start up the bufferd 32K clock ? Does the RTC chip provide a stable 32K clock after startup?

[jemajuca]

Sorry. I mean that the clock would be always available because the RTC chip would be powered by the same power supply than the nRF, but there is a time at the RTC start where the clock is not ready. From NXP datasheet: "...The oscillator is considered to be stopped during the time between power-on and stable crystal resonance. This time can be in the range of 200 ms to 2 s depending on crystal type, temperature, and supply voltage..."

[Sigurd]

In that case it should not be a problem. The SoftDevice will wait for the clock to startup. Remember to specify the correct clock accuracy, e.g. NRF_CLOCK_LF_XTAL_ACCURACY_20_PPM.