nRF52840 spec: external crystal must be enabled for UART - which crystal, 32 MHz or 32.768 kHz?

[Steve Van Kirk]

Hi there,

The spec for nRF52840 says in sec 6.33.1 that for UART, "The external crystal oscillator must be enabled to obtain sufficient clock accuracy for stable communication."  Which external crystal are they referring to, the 32 MHz or the 32.768 kHz? 

I've been through the spec and couldn't tell for sure.  It didn't seem to be stated plainly.  I'm hoping someone with more knowledge than me can provide interpretation or point me to where I missed it in the spec.

Thanks!

Steve

[Einar Thorsrud]

Hi,

This refers to the 32 MHz oscillator, which is what generates the clock for the UART peripheral. If you use the HFINT instead, the baud rate may be too much off (typical accuracy is +- 1.5%, but worst case it can be up to +-8 %, as you can see from Clock chapter in the product specification). In practice, this means that you should explicitly request the HFXO before using the UART and release it once you are done.

[Steve Van Kirk]

Hi Einar,

Thank you for the help - I really appreciate it.

BTW, how did you know the UART clock is generated from the 32 MHz oscillator?  Did I miss that somewhere in the spec.?

Steve

[Einar Thorsrud]

Hi Steve,

It is not clearly stated, but it is the case. You see an indication in the description of the BAUDRATE register though, where you find this line: "Baud rate. Accuracy depends on the HFCLK source selected". (You can also think logically about it, and then it has to be, as the UART baud rate is much higher than the 32.768 kHZ clock, and that is only used by a few low-power peripherals, like the RTC and watchdog).

Einar

[Steve Van Kirk]

Perfect - thanks, Einar.  I just wanted to make sure I wasn't missing something obvious in the spec.  Sounds like I wasn't.

Thanks again for the help!

Steve