External clocks required for USB / UART?

[douglas.malnati]

Hi, I am interested in using the internal RC oscillator on an nRF52840 while also using USB and UART.  Is that supported?

I intend to use UART at 9600 baud.  I'm using USB as well for UART and DFU.

Thanks.

[Susheel Nuguru]

I think lower speed/baudrate I2C/UART can still function with internal HFRC. I have recently tried some simple communication on UART using internal RC HFCLK even at the baudrate of 115200 and I think the tolerance of the serial COM peripherals is much higher than the efficiency lack of internal RC as compared to crystal. 

The way I see it, the peripheral tolerance is directly related to the HFCLK used and I donot see issues with serial peripherals but not use if USB would work properly with internal RC.

[TechGuyMike]

In my experience, USB doesn't work with the internal RC oscillator as the LF clock source on the nRF52840. This is based on my attempts to get the generic HID example to work with the RC oscillator as the LF clock source. But I may be wrong as I've seen no requirements by Nordic for any specific LF clock source for USB to work.