In order to decide whether the IRC of the KEA64 MCU is good enough for my application I have tried to find the accuracy of the bus clock (derived from the system clock) which drives the UARTs. I have a requirement to communicate at 115200bps with an allowed inaccuracy of +/-2%.
Using the IRC->FLL with IRC @ factory trimmed value 31250Hz is not an option as the baudrate will be far off even att room temperature.
So, questions is if the accuracy of the FLL stated in the datasheet (min:-2.3% max:0.8%) is relevant when using an external crystal? OR is the stated accuracy including the IRC inaccuracy of min-0.8% max: 0.8% (min: -1% max:0.8% when trimmed @ 25°C)?
I would also want to know, when changing the IRC frequency through the trim value, is the FLL accuracy affected?
I found the app note https://www.nxp.com/docs/en/application-note/AN5300.pdf which suggest that the FLL output has a guaranteed accuracy of +/-4%. Where does this number come from?