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.

[douglas.malnati]

Oh I also want to use I2C (TWI).

For TWI I see the datasheet indicates the FREQUENCY "Accuracy depends on the HFCLK source selected," so I presume that means the internal RC oscillator will allow I2C to function, but perhaps not at the "true" frequency configured.

Please let me know about the rest, thanks!

[douglas.malnati]

Sorry, I also just noticed that the TEMP sensor is influenced by the HFCLK being an external crystal?

What is the specified impact of using the internal RC oscillator when using the TEMP sensor?

[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.

[Frank Viganske]

Hi,

with the RC oscillator you will get a higher temperature error especially at the extreme temperatures. I had a board showing -53°C at -40°C, and after switching to the crystal it showed -41°C.

If you don't want to measure at very low or high temperatures the RC is probably ok.

Best regards

Frank

[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.