Datasheet
273
XMEGA B [DATASHEET]
8291B–AVR–01/2013
21.8.3 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 an external transmitter is sending using bit rates that are too fast or too slow, or if the internally
generated baud rate of the receiver does not match the external source’s 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.
Table 21-3 and Table 21-4 on page 273 list the maximum receiver baud rate error that can be tolerated. Normal speed
mode has higher tolerance of baud rate variations.
Table 21-3. Recommended maximum receiver baud rate error for normal speed mode.
Table 21-4. Recommended maximum receiver baud rate error for double speed mode.
D 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.
S
F
First sample number used for majority voting. S
F
= 8 for normal speed mode and S
F
= 4 for double speed
mode.
S
M
Middle sample number used for majority voting. S
M
= 9 for normal speed mode and S
M
= 5 for double speed
mode.
R
slow
The ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate.
R
fast
The ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate.
D
#(Data + Parity Bit)
R
slow
[%] R
fast
[%] Max Total Error [%]
Recommended Max
Receiver Error [%]
5 93.20 106.67 +6.67/-6.80 ± 3.0
6 94.12 105.79 +5.79/-5.88 ± 2.5
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
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
R
slow
D 1+S
S 1– DS S
F
++
--------------------------------------------
=
R
fast
D 2+S
D 1+SS
M
+
------------------------------------
=