166
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
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 SF = 4 for Double Speed
mode
S
M
Middle sample number used for majority voting. S
M = 9 for normal speed and SM = 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.
The recommendations of the maximum receiver baud rate error was made under the assumption that the Receiver
and Transmitter equally divides the maximum total error.
There are two possible sources for the receivers baud rate error. The Receiver’s system clock (XTAL) will always
have some minor instability over the supply voltage range and the temperature range. When using a XTAL to gen-
erate the system clock, this is rarely a problem, but for a resonator the system clock may differ more than 2%
depending of the resonators tolerance. The second source for the error is more controllable. The baud rate gener-
ator can not always do an exact division of the system frequency to get the baud rate wanted. In this case an
UBRRn value that gives an acceptable low error can be used if possible.
Table 20-2.
Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 0).
D # (Data+Parity Bit)
R
slow [%]
R
fast [%]
Max. total error [%]
Recommended max. receiver rrror [%]
5
93.20
106.67
+6.67/-6.8
±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
Table 20-3.
Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1).
D # (Data+Parity Bit)
Rslow [%]
Rfast [%]
Max. total error [%]
Recommended max. receiver rrror [%]
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
R
slow
D
1
+
S
1
–
DS
S
F
++
-------------------------------------------
=
R
fast
D
2
+
S
D
1
+
SS
M
+
-----------------------------------
=