HFCLKSTAT and HFCLKSTART and external rail-to-rail clock source

[Vito]

In nrf51422, what is the behavior of these registers when an external rail-to-rail clock source is applied?

1. Do I have to issue HFCLKSTART? If not needed, does issuing it cause some harm?
2. does HFCLKSTAT show the actual state of the external clock? I mean will STATE==RUNNING mean that the external clock is running at the right frequency?

[Jørn]

Hello Vito

If you do not issue a HFCLKSTART the XC1 pin will be high impedance, so you need to issue a HFCLKSTART in order to use the signal generated by the external source.

The running state will show that the HFCLK is currently running, so I suppose it will show that the external oscillator is running. However, I don't think it necessarily means the frequency is correct. The best way to measure if it is correct is by measuring the center frequency on the radio output during transmission. If the external source frequency is wrong, the center frequency will be incorrect.

Best regards

Jørn Frøysa

[Vito]

Thanks. I can assume the external clock frequency will be correct because it comes from a reliable source, but I have to start and stop it with spi commands. So this means that I can use HFCLKSTAT to confirm that the external oscillator started correctly.