31.6.2.3 Clock Generation – Baud-Rate Generator

The baud-rate generator, as shown in Figure 31-3, 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 31-2 contains equations for the baud rate (in bits per second) and the BAUD register value for each operating mode.

For asynchronous operation, there are two different modes: In arithmetic mode, the BAUD register value is 16 bits (0 to 65,535). In fractional mode, the BAUD register is 13 bits, while the fractional adjustment is 3 bits. In this mode the BAUD setting must be greater than or equal to 1.

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

Table 31-2. Baud Rate Equations
Operating Mode	Condition	Baud Rate (Bits Per Second)	BAUD Register Value Calculation
Asynchronous Arithmetic	$f_{B A U D} \leq \frac{f_{r e f}}{S}$	$f_{B A U D} = \frac{f_{r e f}}{S} (1 - \frac{B A U D}{65536})$	$B A U D = 65536 \cdot (1 - S \cdot \frac{f_{B A U D}}{f_{r e f}})$
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. 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:

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

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 table below provides 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 31-3. BAUD Register Value vs. Baud Frequency
BAUD Register Value	Serial Engine CPF	f_BAUD at 48 MHz 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

SAM L21

31.6.2.3 Clock Generation – Baud-Rate Generator

Asynchronous Arithmetic Mode BAUD Value Selection