ATMEGA8_08 Datasheet, PDF (150/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

Table 53. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2X =
0)
D#
(Data+Parity Bit)
5
R_slow(%)
93,20
R_fast(%)
106,67
Max Total
Error (%)
+6.67/-6.8
Recommended Max
Receiver Error (%)
± 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 54. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2X =
1)
D#
(Data+Parity Bit)
5
R_slow(%)
94,12
R_fast(%)
105,66
Max Total
Error (%)
+5.66/-5.88
Recommended Max
Receiver Error (%)
± 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
The recommendations of the maximum Receiver baud rate error was made under the assump-
tion 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 tempera-
ture 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 2% 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 UBRR value
that gives an acceptable low error can be used if possible.
150 ATmega8(L)
2486T–AVR–05/08