34.5.2.3 Clock Generation – Baud-Rate Generator

Each SERCOM contains a baud rate generator as shown in the following figure, which can generate an internal clock for asynchronous or synchronous communication. The output frequency (f_BAUD) is determined by the Baud register (BAUD) setting and the baud reference clock 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.

The following table contains equations for the baud rate (in bits per second) and the BAUD register value for each operating mode.

For Asynchronous Arithmetic mode, the BAUD register value is 16 bits (0 to 65,535).

For Asynchronous Fractional mode, the BAUD register value contains 13 bits of integer (BAUD.BAUD) as well as a 3 bit fractional adjustment (BAUD.FP). In this mode the BAUD setting must be greater than or equal to 1. This mode is used for auto baud detection. See CTRLA.FORM and CRTLA.SAMPR.

For Synchronous operation, the BAUD register value is 8 bits (0 to 255). CRTLA.CMODE=1.

Table 34-2. Baud Rate Equations
Operating Mode	Condition	Baud Rate (Bits Per Second)	BAUD Register Value Calculation
Asynchronous Arithmetic	$f_{B A U D} \leq \frac{f_{r e f}}{16}$	$f_{B A U D} = \frac{f_{r e f}}{16} (1 - \frac{B A U D}{65536})$	$B A U D = 65536 \cdot (1 - 16 \cdot \frac{f_{B A U D}}{f_{r e f}})$
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 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})

Asynchronous Arithmetic Mode BAUD Value Selection

The formula given for f_BAUD calculates the average frequency over 65536 f_ref cycles. Although the BAUD register can be set to any value between 0 and 65536, the actual average frequency of f_BAUD over a single frame is more granular. The BAUD register values that will affect the average frequency over a single frame lead to an integer increase in the cycles per frame (CPF)

C P F = \frac{f_{r e f}}{f_{B A U D}} (D + S)

where:

D represent the data bits per frame
S represent the sum of start and first stop bits, if present.

The following table shows the BAUD register value versus baud frequency f_BAUD at a serial engine frequency of 48 MHz. This assumes a D value of 8 bits and an S value of 2 bits (10 bits, including start and stop bits).

Table 34-3. BAUD Register Value vs. Baud Frequency
BAUD Register Value	Serial Engine CPF	f_BAUD at 48MHz Serial Engine Frequency (f_REF)
0 – 406	160	3 MHz
407 – 808	161	2.981 MHz
809 – 1205	162	2.963 MHz
...	...	...
65206	31775	15.11 kHz
65207	31871	15.06 kHz
65208	31969	15.01 kHz