Current Consumption For Using Digital LF Clock (non-crystal)

[mcintoda]

I am using an RTC for other purposes that exposes a buffered 32kHz clock. I am considering using the buffered (rail-to-rail) for also the LF timebase of the nRF51. The drive current when using crystal oscillator is ~1uA.

Do we still consume this current if a rail-rail clock is applied to XL1 or does internal AGC reduce current drive?

For calculating capacitive losses with rail-to-rail drive is 4pF input correct assumption?

P_XIN = 1/2 * C * V^2 * 32.767kHz = 1/2 * 4pF * 3V^2 *32.768kHz

[Asbjørn]

The 32 kHZ XOSC will still be running, so typically it will consume 0.4 uA during run time even when you are feeding it with an external clock.

The capacitance will depend on the board stack up and length of the trace as well, but normally I would say 4 pF is well within the ballpark of what you would use. So it's a good assumption. If you have long traces or have a close ground reference around the trace, this value might be higher.