ATMEGA8_14 Datasheet, PDF (144/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

ATmega8(L)
Asynchronous
Operational Range
The operational range of the Receiver is dependent on the mismatch between the received bit
rate and the internally generated baud rate. If the Transmitter is sending frames at too fast or too
slow bit rates, or the internally generated baud rate of the Receiver does not have a similar (see
Table 53) base frequency, the Receiver will not be able to synchronize the frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal
Receiver baud rate.
R_slow = _S-----–----₁--^--^D-+-----+_D-----¹-_---^_S--^S---+----_S----_F-
R_fast = -_--_D----^--+-^D----₁--+-_---_S²----^_+---^S--_S---_M---
D Sum of character size and parity size (D = 5-bit to 10-bit)
S Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode
S_F First sample number used for majority voting. S_F = 8 for Normal Speed and S_F = 4 for
Double Speed mode
S_M Middle sample number used for majority voting. S_M = 9 for Normal Speed and S_M = 5 for
Double Speed mode
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
Table 53 and Table 54 list the maximum Receiver baud rate error that can be tolerated. Note
that Normal Speed mode has higher toleration of baud rate variations.
Table 53. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2X = 0)
D#
(Data + Parity Bit)
R_slow
(%)
R_fast
(%)
Max Total
Error (%)
Recommended Max
Receiver Error (%)
5
93.20 106.67
+6.67/-6.8
±3.0
6
94.12 105.79
+5.79/-5.88
±2.0
7
94.81 105.11
+5.11/-5.19
±2.0
8
95.36 104.58
+4.58/-4.54
±2.0
9
95.81 104.14
+4.14/-4.19
±1.5
10
96.17 103.78
+3.78/-3.83
±1.5
2486AA–AVR–02/2013
Table 54. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode
(U2X = 1)
D#
(Data + Parity Bit)
R_slow
(%)
R_fast
(%)
Max Total
Error (%)
Recommended Max
Receiver Error (%)
5
94.12 105.66 +5.66/-5.88
±2.5
6
94.92 104.92 +4.92/-5.08
±2.0
7
95.52 104.35 +4.35/-4.48
±1.5
8
96.00 103.90 +3.90/-4.00
±1.5
9
96.39 103.53 +3.53/-3.61
±1.5
10
96.70 103.23 +3.23/-3.30
±1.0
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