31.6.2.6.4 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 is depending 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 will not be able to synchronize the frames to the start bit.

There are two possible sources for a mismatch in baud rate: First, the reference clock will always have 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 should be set to give the lowest possible error. Refer to Clock Generation – Baud-Rate Generator for details.

Recommended maximum receiver baud-rate errors for various character sizes are shown in the table below.

Table 31-3. Asynchronous Receiver Error for 16-fold Oversampling
D 
(Data bits+Parity) RSLOW [%] RFAST [%] 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 ⁢ =   ⁢ ( D ⁢ +   1 ) S S − 1 + D ⋅   S + S F , R F A S T ⁢ =   ( D ⁢ +   2 ) S ( D + 1 ) S + S M
  • RSLOW is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate
  • RFAST 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 character size and parity size (D = 5 to 10 bits)
  • S is the number of samples per bit (S = 16, 8 or 3)
  • SF is the first sample number used for majority voting (SF = 7, 3, or 2) when CTRLA.SAMPA=0.
  • SM is the middle sample number used for majority voting (SM = 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:
Figure 31-5. USART Rx Error Calculation
The recommendation values in the table above accommodate errors of the clock source and the baud generator. The following figure gives an example for a baud rate of 3Mbps:
Figure 31-6. USART Rx Error Calculation Example