nRF51422 oscillator

[Ehsan]

Hi,

I have been using the nRF51-DK to develop a program for capacitive sensing. I want to design a custom PCB with the nRF51422 chip on it. Do I need to have an oscillator/crystal on my PCB to provide a clock or does the chip come with an internal oscillator? Also, what oscillator does the nRF51-DK use by default?

Regards, Ehsan.

[Sigurd]

Hi,

The nRF51422 have two internal RC Oscillators (16 MHz and 32.768 kHz). The internal RC isn't accurate enough for radio operation, so if you are planning on using Bluetooth you need an external 16 MHz crystal that have at least +-40 ppm accuracy or better.

On the nRF51-DK there is both an external 16 MHz (+- 15ppm) and 32kHz (+- 20ppm) crystal.

Since you are designing your own custom PCB I would recommend taking a look at the General PCB design guidelines for nRF51 series tutorial.

[Ehsan]

Does the nRF51-DK use the 32.768 kHz internal RC oscillator by default? Thanks

[Sigurd]

The CPU is driven by the HFCLK(high frequency clock), and the HFCLK is driven by either:

• External crystal oscillator : 16 or 32 MHz crystal oscillator

OR

• Internal RC oscillator : 16 MHz RC oscillator

The RTC, and low-power timers uses the LFCLK (either driven by 32.768 kHz external crystal or internal RC).

I recommend that you take a look at chapter 13 in the Reference Manual. (Take a look at Figure 13 at page 51)

[Sigurd]

Q: Does the nRF51-DK use the 32.768 kHz internal RC oscillator by default?

A: If you want to use the internal RC oscillator or external crystal oscillator is something you choose in your application when you start the LFCLK. By default the LFCLK will initially start running from the internal 32.768 kHz RC oscillator while the external crystal oscillator is starting up and automatically switch to using the external crystal oscillator once this oscillator is running. (If you have chosen to use internal RC oscillator, there will of course not be a switch to the external crystal oscillator)