nRF52840 External 32.768kHz crystall

[Nikosant03]

Hi everyone, 

I am desing my own pcb using nRF52840 SOC. Looking the reference design I can see two external crystalls one  32.768kHz and one 32MHz.

1. The 32.768kHz is used by the RTC isn't? So without this crystal I cannot use the RTC and therefore the BLE, because the softdevice uses the RTC0. If this is true why is it mentioned as optional on the reference circuit (see section 7.3 of nRF52840 Product Specification)?

2. What is the purpose of 32Mhz crystal? Is it vital?

Thanks in advance

Nick

[haakonsh]

If this is true why is it mentioned as optional on the reference circuit

 It's not true, there's an internal 32.768kHz RC oscillator that you can use. 

What is the purpose of 32Mhz crystal? Is it vital?

It is required if you intend to use the RADIO peripheral. The 2.4GHz radio requires a very accurate clock source for its PLL. 

[Nikosant03]

Thanks for your reply,

What is the purpose of the external 32.768kHz clock if I can use the internal one? If it is something that is not critical I am considering of ommiting from my design.

[haakonsh]

The crystal has lower current consumption and it is more accurate and stable over temperature. 

[Nikosant03]

Understood, thank for your support!!