
140
ATmega8A [DATASHEET]
8159E–AVR–02/2013
generated baud rate of the Receiver does not have a similar (see
Table 20-2) 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.
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
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 20-2 and
Table 20-3 list the maximum Receiver baud rate error that can be tolerated. Note that Normal
Speed mode has higher toleration of baud rate variations.
Table 20-2.
Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2X = 0)
D#
(Data+Parity Bit)
Rslow(%)
Rfast(%)
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
Table 20-3.
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
Rslow
D 1
+
S
S 1
–
DS
SF
++
-------------------------------------------
=
Rfast
D 2
+
S
D 1
+
SS
M
+
-----------------------------------
=