45.6.1.2 Fractional Baud Rate in Asynchronous Mode

The baud rate generator is subject to the following limitation: the output frequency changes only by integer multiples of the reference frequency. An approach to this problem is to integrate a fractional N clock generator that has a high resolution. The generator architecture is modified to obtain baud rate changes by a fraction of the reference source clock. This fractional part is programmed using US_BRGR.FP. If FP is not 0, the fractional part is activated. The resolution is one-eighth of the clock divider. The fractional baud rate is calculated using the following formula:

Baud Rate = \frac{Selected Clock}{(8 (2 - OVER) (CD + \frac{FP}{8}))}

The modified architecture is presented in the following figure.

Figure 45-3. Fractional Baud Rate Generator

Warning: When the value of US_BRGR.FP is greater than '0', the SCK (oversampling clock) generates non-constant duty cycles. The SCK high duration is increased by “selected clock” period from time to time. The duty cycle depends on the value of USART_BRGR.CD.