Table 74. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2Xn = 0)
D
Recommended Max
Receiver Error (%)
# (Data+Parity Bit)
R
slow (%)
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 75. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode
(U2Xn = 1)
D
Recommended Max
Receiver Error (%)
# (Data+Parity Bit) Rslow (%) Rfast (%) Max Total Error (%)
5
6
94.12
94.92
95.52
96.00
96.39
96.70
105.66
104.92
104,35
103.90
103.53
103.23
+5.66/-5.88
+4.92/-5.08
+4.35/-4.48
+3.90/-4.00
+3.53/-3.61
+3.23/-3.30
2.5
2.0
1.5
1.5
1.5
1.0
7
8
9
10
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 crystal to generate the system clock, this is rarely
a problem, but for a resonator the system clock may differ more than 2ꢀ depending of
the resonators tolerance. The second source for the error is more controllable. The baud
rate generator 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.
178
ATmega48/88/168
2545D–AVR–07/04
ATMEL [ ATMEL ]
