32.6.2.6.4 Asynchronous Operational Range

The operational range of the asynchronous reception depends on the accuracy of the internal baud-rate clock, the rate of the incoming frames and the frame size (in number of bits). In addition, the operational range of the receiver depends on the difference between the received bit rate and the internally-generated baud rate. If the baud rate of an external transmitter is too high or too low compared to the internally-generated baud rate, the receiver is not able to synchronize the frames to the start bit.

There are two possible sources for a mismatch in the baud rate:

First, the reference clock always has some minor instability.
Second, the baud-rate generator cannot always do an exact division of the reference clock frequency to get the baud rate desired.

In this case, the BAUD register value must be set to give the lowest possible error. See Clock Generation – Baud-Rate Generator from Related Links.

The following table provides an overview of the recommended maximum receiver baud-rate errors for various character sizes.

Table 32-3. Asynchronous Receiver Error for 16-fold Oversampling
D  (Data Bits+Parity)	R_SLOW [%]	R_FAST [%]	Max. Total Error [%]	Recommended Max. Rx Error [%]
5	94.12	107.69	+5.88/-7.69	±2.5
6	94.92	106.67	+5.08/-6.67	±2.0
7	95.52	105.88	+4.48/-5.88	±2.0
8	96.00	105.26	+4.00/-5.26	±2.0
9	96.39	104.76	+3.61/-4.76	±1.5
10	96.70	104.35	+3.30/-4.35	±1.5

The following equations calculate the ratio of the incoming data rate and internal receiver baud rate:

R_{S L O W} = \frac{(D + 1) S}{S - 1 + D \cdot S + S_{F}},

R_{F A S T} = \frac{(D + 2) S}{(D + 1) S + S_{M}}

R_SLOW is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate.
R_FAST is the ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate.
D is the sum of the character size and parity size (D = 5 to 10 bits).
S is the number of samples per bit (S = 16, 8 or 3).
S_Fis the first sample number used for majority voting (S_F = 7, 3 or 2) when CTRLA.SAMPA = 0.
S_Mis the middle sample number used for majority voting (S_M = 8, 4 or 2) when CTRLA.SAMPA = 0.

The recommended maximum RX Error assumes that the receiver and transmitter equally divide the maximum total error. Its connection to the SERCOM Receiver error acceptance is depicted in this figure:

The recommendation values in the table above accommodate errors of the clock source and the baud generator. The following figure illustrates an example for a baud rate of 3 Mbps:

Figure 32-6. USART RX Error Calculation Example