16MHz RC oscillator accurate enough for UART

[Remo]

Hi all

According the nRF51822 Product Specification V3.1, the 16MHz RC oscillator has a tolerance of at most 5% and can be used as clock source for the UART module.

Due to several sources on the internet, the maximum tolerance for a proper UART communication is 4%. This would give 2% to the transmitter and 2% to the receiver.

Therefore, is the RC oscillator really accurate enough? Am I missing something?

Kind regards

Remo

[Susheel Nuguru]

When using RC oscillator, if you are using hardware flow control, then i have not seen any issues even using high baudrate. But if you are not using hardware flow control then there can be framing errors with baudrate starting 38400.

It is highly recommended to use XTAL when not using flow control.

[Remo]

What does the use of the hardware flow control have to do with an unaccurate clock source? Even if flow control is used, it could be possible that the bits are sampled/sent erroneous due to the wrong clock, or not?

We are using 38400 and 115200 without flow control. This means we have to enable the XTAL? If the XTAL is requested with the function sd_clock_hfclk_request(), is guaranteed that the SoftDevice doesn't disable the XTAL by itself?

Is this "recommendation" documented somewhere? Because I've nothing found in the reference manual and the product specification.