ATMEGA32 Datasheet, PDF (154/317 Pages) ATMEL Corporation – 8-bit AVR Microcontroller with 32K Bytes In-System Programmable Flash

Table 61 and Table 62 list the maximum receiver baud rate error that can be tolerated.
Note that Normal Speed mode has higher toleration of baud rate variations.
Table 61. 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
± 2.5
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 62. 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
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 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.
154 ATmega32(L)
2503F–AVR–12/03