Should I use a 32KHz watch crystal or 16MHz crystal

[John Perez]

I am designing with the nRF51822 and I have a choice of either a 32Khz or 16MHz crystal. Can you tell me the advantages and disadvantages of both?

If the 32KHz crystal offers lower power and higher accuracy why even offer the 16MHz option? Also, it seems to me that the 16MHz would offer higher accuracy since the PLL multiplier would be smaller than for a watch crystal. Since the PLL multiplier would be smaller the effects of jitter multiplication would be less. Or am I missing something?

Thanks, john

[TT]

Here's the basic concept:

1. the 32KHz it's for RTC (Real Time Clock) only, for example after RX/TX the you enter to sleep mode, and wake up after a giving time that you want (IRQ from the RTC), while the 32KHz RTC runs it consume a little power, and that's help to reduce the power consumption of the chip by setting known on/off working time. the nrf51822 have an option to use internal RTC or synt. RTC (created from the 16MHz) but it consume much more than external RTC. (please see the datasheet for more information about the power consumption)

2. the 16MHz used for MCU core, and also for the RF core (by using PLL you have the 2.4GHz Signal),

in summery, you need the 16MHz, about the 32KHz you can use internal/external (your chose: power vs. board space vs. cost reduction)

[John Perez]

Thanks for clearing this up.

[Jay Tyzzer]

Hi John,

There are three ways to generate the 32khz clock:

1) External Crystal

a) Pro: Lower power consumption. Can be used for other timers if critical timing is required. The BTLE stack itself does not need the accuracy of the crystal but the ANT stack does!

b) Con: Extra cost and board space ( The crystal Plus load caps)

2) Using the internal 32Khz RC osc. / 16Mhz clock calibration method

a) Pro: Lower BOM cost and Board space reduction. Can be adjusted to recalibrate from 4000ms to 16000ms and can use Die temp to determine when recalibration is required.

b) Con: Increased power consumption over #1 above

**3) Low frequency clock synthesized Internally by dividing down the High Freq 16Mhz clock **

a) Con: Not recommended for use with any of the Nordic BTLE Softdevices. 16Mhz Crystal must run all the time to generate 32Khz clock.

Or if a stable 32Khz clock is available from a external source then this may be used as well. This clock must meet the specs shown in the datasheet.