Can I share both crystals on nRF8001

[Fred Roeber]

Hi, I am using the nRF8001 chip with an external MCU that has both 16MHz and 32KHz crystals. I saw the app note about sharing the 16MHz crystal between MCU and radio. And the nRF8001 data sheet talks about sharing the 16MHz crystal. But there is no mention of sharing the 32KHz crystal. Can this be done and what are the parameters if I do it (like how much load the crystal input adds to the external signal?

[David Edwin]

You can provide a 32Khz source externally as an analog or digital source or use the internal 32Khz oscillator. It should be possible to share the 32KHz so long as it is kept running all the time when the nRF8001 radio is actively advertising or in a connection, ideally you supply the 32kHz whenever the nRF8001 is not sleeping. You need to wait for 50ms after calling the nRF8001 sleep command before cutting off the 32Khz.

Using the schematic and BOM in section 17.4 of the nRF8001 v1.2 datasheet: The cap (15pF x 2) values on XL1 and XL2 are related to the crystal used.

When you are sharing the 32KHz crystal , you will connect both ends of crystal to mcu first and then connect only the XL1 of nRF8001. I am thinking that a 12pF load on the line that is on XL1 and mcu would be ok. The cap on the other side of the 32KHz can be the same 15pF. XL2 of the nRF8001 is not connected.

You may need to iteratively find better values here. If you need more help you can jump to the support portal.

[David Edwin]

Pretty sure that the 32kHz input can be taken by the nRF8001 as a digital source. You should be able to share the 16Mhz as well by running it all the time but I think a more nuanced answered can be done on the support portal.