Atmel AT90PWM2 Manual page 199

18.8.3 Asynchronous Operational Range
AT90PWM2/3/2B/3B
200
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 18-2) 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 Sum of character size and parity size (D = 5 to 10 bit)
S Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed
mode.
S First sample number used for majority voting. S
F
for Double Speed mode.
S Middle sample number used for majority voting. S
M
S = 5 for Double Speed mode.
M
R is the ratio of the slowest incoming data rate that can be accepted in relation to the
slow
receiver baud rate.
R is the ratio of the fastest incoming data rate that can be accepted in relation to the
fast
receiver baud rate.
Table 18-2 and Table 18-3
that Normal Speed mode has higher toleration of baud rate variations.
Table 18-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2X = 0)
D
# (Data+Parity Bit) R
5
6
7
8
9
10
)S
1
D +
D S ⋅
+ + S
F
list the maximum receiver baud rate error that can be tolerated. Note
(%) R (%) Max Total Error (%)
slow fast
93.20 106.67
94.12 105.79
94.81 105.11
95.36 104.58
95.81 104.14
96.17 103.78
( )S
2
D +
R = -----------------------------------
( )S
fast
1
D + + S
= 8 for normal speed and S
F
= 9 for normal speed and
M
Recommended Max
Receiver Error (%)
+6.67/-6.8
+5.79/-5.88
+5.11/-5.19
+4.58/-4.54
+4.14/-4.19
+3.78/-3.83
M
= 4
F
± 3.0
± 2.5
± 2.0
± 2.0
± 1.5
± 1.5
4317I–AVR–01/08