25.6.2.3 Clock Generation – Baud-Rate Generator

The baud-rate generator, as shown in Figure 25-3, generates internal clocks for asynchronous and synchronous communication. The output frequency (f_BAUD) is determined by the Baud register (BAUD) setting and the baud reference frequency (f_ref). The baud reference clock is the serial engine clock, and it can be internal or external.

For asynchronous communication, the /16 (divide-by-16) output is used when transmitting, whereas the /1 (divide-by-1) output is used while receiving.

For synchronous communication, the /2 (divide-by-2) output is used.

This functionality is automatically configured, depending on the selected operating mode.

Table 25-2 contains equations for the baud rate (in bits per second) and the BAUD register value for each operating mode.

For asynchronous operation, there is one mode: arithmetic mode, the BAUD register value is 16 bits (0 to 65,535).

For synchronous operation, the BAUD register value is 8 bits (0 to 255).

Table 25-2. Baud Rate Equations
Operating Mode	Condition	Baud Rate (Bits Per Second)	BAUD Register Value Calculation
Asynchronous Arithmetic	$f_{B A U D} \leq \frac{f_{r e f}}{16}$	$f_{B A U D} = \frac{f_{r e f}}{16} (1 - \frac{B A U D}{65536})$	$B A U D = 65536 \cdot (1 - S \cdot \frac{f_{B A U D}}{f_{r e f}})$
Asynchronous Fractional	$f_{B A U D} \leq \frac{f_{r e f}}{S}$	$f_{B A U D} = \frac{f_{r e f}}{S \cdot (B A U D + \frac{F P}{8})}$	$B A U D = \frac{f_{r e f}}{{S \cdot f}_{B A U D}} - \frac{F P}{8}$
Synchronous	$f_{B A U D} \leq \frac{f_{r e f}}{2}$	$f_{B A U D} = \frac{f_{r e f}}{2 \cdot (B A U D + 1)}$	$B A U D = \frac{f_{r e f}}{{2 \cdot f}_{B A U D}} - 1$

S - Number of samples per bit, which can be 16, 8, or 3.

The Asynchronous Fractional option is used for auto-baud detection.

The baud rate error is represented by the following formula:

E r r o r = 1 - (\frac{E x p e c t e d B a u d R a t e}{A c t u a l B a u d R a t e})

SAM D21/DA1 Family

25.6.2.3 Clock Generation – Baud-Rate Generator