27.3.3.2.3 Error Tolerance

The speed of the internally generated baud rate and the externally received data rate has to be identical, but, due to natural clock source error, this is usually not the case. The USART is tolerant of such error, and the limits of this tolerance make up what is sometimes known as the Operational Range.

The following tables list the operational range of the USART, being the maximum receiver baud rate error that can be tolerated. Note that Normal-Speed mode has higher toleration of baud rate variations than Double-Speed mode.

Table 27-4. Recommended Maximum Receiver Baud Rate Error for Normal-Speed Mode
D Rslow [%] Rfast [%] Maximum Total Error [%] Recommended Max. Receiver Error [%]
5 93.20 106.67 -6.80/+6.67 ±3.0
6 94.12 105.79 -5.88/+5.79 ±2.5
7 94.81 105.11 -5.19/+5.11 ±2.0
8 95.36 104.58 -4.54/+4.58 ±2.0
9 95.81 104.14 -4.19/+4.14 ±1.5
10 96.17 103.78 -3.83/+3.78 ±1.5
Note:
  • D: The sum of character size and parity size (D = 5 to 10 bits)
  • RSLOW: The ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate
  • RFAST: The ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate
Table 27-5. Recommended Maximum Receiver Baud Rate Error for Double-Speed Mode
D Rslow [%] Rfast [%] Maximum Total Error [%] Recommended Max. Receiver Error [%]
5 94.12 105.66 -5.88/+5.66 ±2.5
6 94.92 104.92 -5.08/+4.92 ±2.0
7 95.52 104.35 -4.48/+4.35 ±1.5
8 96.00 103.90 -4.00/+3.90 ±1.5
9 96.39 103.53 -3.61/+3.53 ±1.5
10 96.70 103.23 -3.30/+3.23 ±1.0
Note:
  • D: The sum of character size and parity size (D = 5 to 10 bits)
  • RSLOW: The ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate
  • RFAST: The ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate

The recommendations of the maximum receiver baud rate error were made under the assumption that the receiver and transmitter equally divide the maximum total error.

The following equations are used to calculate the maximum ratio of the incoming data rate and the internal receiver baud rate.

R S L O W = S ( D + 1 ) S ( D + 1 ) + S F − 1
R F A S T = S ( D + 2 ) S ( D + 1 ) + S M
  • D: The sum of character size and parity size (D = 5 to 10 bits)
  • S: Samples per bit. S = 16 for Normal-Speed mode and S = 8 for Double-Speed mode.
  • SF: First sample number used for majority voting. SF = 8 for Normal-Speed mode and SF = 4 for Double-Speed mode.
  • SM: Middle sample number used for majority voting. SM = 9 for Normal-Speed mode and SM = 5 for Double-Speed mode.
  • RSLOW: The ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate
  • RFAST: The ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate