ATmega169P
The following equations can be used to calculate the ratio of the incoming data rate and internal
receiver baud rate.
(D + 1)S
S – 1 + D ⋅ S + S
(D + 2)S
(D + 1)S + S
R
= ------------------------------------------
R
= -----------------------------------
slow
fast
F
M
D
S
Sum of character size and parity size (D = 5 to 10 bit)
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 and SF = 4
for Double Speed mode.
SM
Middle sample number used for majority voting. SM = 9 for normal speed and
SM = 5 for Double Speed mode.
Rslow
is the ratio of the slowest incoming data rate that can be accepted in relation to the
receiver baud rate. Rfast is the ratio of the fastest incoming data rate that can be
accepted in relation to the receiver baud rate.
Table 18-2 and Table 18-3 list the maximum receiver baud rate error that can be tolerated. Note
that Normal Speed mode has higher toleration of baud rate variations.
Table 18-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2Xn = 0)
D
Recommended Max
Receiver Error (%)
# (Data+Parity Bit)
Rslow (%)
93.20
94.12
94.81
95.36
95.81
96.17
Rfast (%)
106.67
105.79
105.11
104.58
104.14
103.78
Max Total Error (%)
+6.67/-6.8
5
6
3.0
2.5
2.0
2.0
1.5
1.5
+5.79/-5.88
+5.11/-5.19
+4.58/-4.54
+4.14/-4.19
+3.78/-3.83
7
8
9
10
Table 18-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode
(U2Xn = 1)
D
Recommended Max
Receiver Error (%)
# (Data+Parity Bit)
Rslow (%)
94.12
94.92
95.52
96.00
96.39
96.70
Rfast (%)
105.66
104.92
104,35
103.90
103.53
103.23
Max Total Error (%)
+5.66/-5.88
5
6
2.5
2.0
1.5
1.5
1.5
1.0
+4.92/-5.08
7
+4.35/-4.48
8
+3.90/-4.00
9
+3.53/-3.61
10
+3.23/-3.30
187
8018A–AVR–03/06
ATMEL [ ATMEL ]
