Crystal part number selection

[kadam]

Dear sir,

I am new to nRF52840 IC. We have selected two part number for 32.768KHz crystal.
1) ECS-.327-7-34B-TR(www.digikey.in/.../1850390)
2) ABS07-32.768KHZ-T(www.digikey.com/.../1237009)
Which part number provide low current consumption and better start up?

[ketiljo]

The crystal with the lowest CL value will draw less current and start faster. The ECS part have lower CL value than the the ABS part. 

[kadam]

Dear sir,

Thank you for your reply.

Please guide us for how 32.768KHz crystal is important for ANT protocol. How we can select crystal part number?

[Kenneth]

You can find the crystal requirements here:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/clock.html?cp=4_0_0_4_3_3_2#unique_1547564644

As Ketil writes, it typically preferred to use a crystral with lower CL value.

If you choose a crystal that is out of specifications you will have issues maintain a link, since they will not be able to maintain synchronization.

[kadam]

Dear sir,

As per nRF52840 datasheet, maximum load capacitance require is 12.5pF. So we select crystal ABS07-32.768KHz-T. We have follow equation as per nRF52840 datasheet and done calculation. For Stray capacitance we have follow 0.8pF per centimeter as per TI document(http://www.ti.com/lit/an/slla051/slla051.pdf). I have attached calaculation. Kindly suggest we have follow correct method or not. And please guide us it is better to use 7pF or 12pF.How to Choose Load Capacitor values for oscillator.docx

[ketiljo]

Its says MAX load is 12.5 pF, meaning both will work. But again, ECS part will be better. 

Ccap = CL * 2 - Cpin - Ctrace. Round down to the nearest value. 

For a 12 pF crystal, the load caps should be 18 pF

For a 7 pF crystal, the load caps should be 12 pF

[kadam]

Dear sir,

Thank you for your reply.