nRF52832 external crystals needed or not

[Mattias]

Hello,

I need some clarifications about external crystals for nRF52832.

We are evaluating a prototype system with two units A and B each containing an nRF52832. A and B will communicate via UART. Unit A will also use BLE to communicate with other units. Unit A shall wake up on RTC every 10 ms to check if UART is connected to unit B.
Unit B are powered from unit A so will be an POR every time unit A is connected.

1. Unit A needs a 32 MHz crystal since it will use BLE, correct?
2. Unit A needs a 32 kHz crystal for the RTC wakeup, correct?
3. We don't need a 32 kHz crystal for unit B, correct?
4. Do we need a 32 MHz crystal for unit B? According to the answer here we don't need a crystal at all since we are not using BLE but according to the design guide lines found here the 32 MHz crystal is mandatory on nRF52832. I know the question in the first link is about nRF51 but the answer indicates that nRF52/nRF51 works in the same way.

The main reason to exclude crystals in unit B is to save space.

Thanks in advance, Mattias

[RK]

1. yes
2. no - but you'll get possibly bad RTC accuracy with the LC oscillator. If you don't care that 10ms becomes 12ms, that's not an issue.
3. you do not
4. you do not, the unit will run off RC for as long as you like. Might find UART timing at high baud rates a bit interesting, but it'll run

you can test all this stuff with the dev kit by just writing code which doesn't even turn the LF or HF crystal oscillators on.