# Take advantage of external RTC to avoid 32.768KHz duplicity?

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.

edit retag close delete

Sort by » oldest newest most voted

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.

more

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.

( 2017-01-13 10:04:25 +0100 )editconvert to answer

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?

( 2017-01-13 15:13:45 +0100 )editconvert to answer

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..."

( 2017-01-13 17:36:38 +0100 )editconvert to answer

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.

( 2017-01-16 12:46:46 +0100 )editconvert to answer

Hello Sigurd. It's OK. I will check the overall power comsumption with this solution. Thankyou.

more

[hide preview]

## Recent blog posts

• ### Difference between nRF52840 Engineering A and B reference designs

Posted 2018-01-15 12:27:08 by Martin Børs-Lind
• ### [For Hire] Expert development services of custom Hardware devices | IoT solutions | Mobile Apps

Posted 2018-01-15 09:08:42 by Ilya Surinsky
• ### Rust bindings to nrf52 series

Posted 2018-01-12 23:23:07 by Coleman McFarland
• ### Show And Tell: Poly - Building an RC Airplane The Hard Way

Posted 2018-01-05 01:17:57 by Daniel Veilleux
• ### Bluetooth on a desktop computer (Windows, Mac, Linux)

Posted 2018-01-04 17:56:57 by kbaud

## Recent questions

• ### System off draws more then 80µA

Posted 2018-01-22 22:15:19 by ruetten
• ### How to add driver to an SDK example, in Segger Embedded Studio?

Posted 2018-01-22 21:37:18 by Mark Leavitt
• ### Send more than 60 characters with each call to ble_nus_string_send

Posted 2018-01-22 21:20:21 by fkoran
• ### DFU succeeds but flash is erased

Posted 2018-01-22 20:42:54 by brett_anderson
• ### Uart example stops blinking after a while

Posted 2018-01-22 19:56:29 by oliverpoc