Rainbow Electronics ATmega128L - Manual (Page 181)

181
ATmega128(L)
2467B–09/01
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.
Multi-processor
Communication Mode Setting the Multi-processor Communication Mode (MPCM) bit in UCSRA enables a fil-
tering function of incoming frames received by the USART receiver. Frames that do not
contain address information will be ignored and not put into the receive buffer. This
effectively reduces the number of incoming frames that has to be handled by the CPU,
in a system with multiple MCUs that communicate via the same serial bus. The transmit-
ter is unaffected by the MPCM setting, but has to be used differently when it is a part of
a system utilizing the Multi-processor Communication Mode.
If the receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop
bit indicates if the frame contains data or address information. If the receiver is set up for
frames with 9 data bits, then the 9th bit (RXB8) is used for identifying address and data
Table 75. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2X = 0)
D
# (Data+Parity Bit) R
slow
% R
fast
%
Max Total
Error %
Recommended Max
Receiver Error %
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 76. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode
(U2X = 1)
D
# (Data+Parity Bit) R
slow
% R
fast
%
Max Total
Error %
Recommended Max
Receiver Error %
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