16.2.1. Clock signals

The UARTs can be clocks from an internal reference or the external reference clock UARTCLKEXT can be selected.

The frequency selected for UARTCLK must accommodate the required range of baud rates:

FUARTCLK (min) >= 16 x baud_rate (max)

FUARTCLK(max) <= 16 x 65535 x baud_rate (min)

For example, for a range of baud rates from 110–460 800 baud the UARTCLK frequency must be within the range 7.3 728–115MHz.

The frequency of UARTCLK must also be within the required error limits for all baud rates to be used.

There is also a constraint on the ratio of clock frequencies for PCLK to UARTCLK. The frequency of UARTCLK must be no more than 5/3 times faster than the frequency of PCLK:


This gives sufficient time to write the received data to the receive FIFO.

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