UART reading size

That’s not the way FCM works - you must write your application to accept the data in whatever arbitrary size FCM may choose.

See: Q2686 UART receive buffer size is limit only 56? - #10 by awneil

And: FCM and WIP - #10 by awneil

Here’s an example of how to handle data arriving in arbitrary-sized blocks: Good paper on Event-Driven Programming - #6 by awneil