[consideration] 32k MEMS Oscillator for little board space, high stability, low power?

[bvrm]

I consider to use a 32k MEMS oscillator instead of a usual 32k quartz crystal with a nRF51822 (WLCS). 32k MEMS oscillator's package is quite small 1.55 x 0.85 mm, there's no need for load capacitors. So it's easy to apply. Frequency stability is 10 ppm initial and <= 100 ppm over temperature without the need for load capacitor tuning. Supply current is about 1µA low swing and 1.6µA full swing mode.

nRF51 reference manual 12.1.1 gives two options for alternative clock sources:

1. A rail-to-rail clock signal
2. A low swing clock signal

There is no values in the nRF51822 product specification 8.1 for alternative clock sources:

• What is the clock source current of the nRF51 for both options?

• What are the low swing clock signal voltages (V_high/V_low) needed for proper operation? The smaller the low swing voltages the lower the current consumption of the MEMS oscillator.

• Are the internal oscillators (pierce and RC) automatically disabled when feeding an external clock?

[bvrm]

No idea on this topic?

[Håkon Alseth]

If you follow the recommendations given here, it'll work: devzone.nordicsemi.com/.../

You can use the normal external xtal source for starting it. Remember to input the tolerance (macros with different ppm's) when using the softdevice.

The swing should be greater than 200 mV. If you're going to use rail-to-rail, you must make sure that the voltage it outputs is within VDD_NRF+0.3 and 0-0.3V (the maximum ratings for the chip)

Cheers, Håkon