Datasheet

D
# (Data+Parity Bit)
R
slow
[%] R
fast
[%] Max. Total Error [%] Recommended Max. Receiver Error [%]
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 21-3. 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
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 (EXTCLK)
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 resonator's 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.
21.10. Multi-Processor Communication Mode
Setting the Multi-Processor Communication mode (MPCMn) bit in UCSRnA enables a filtering 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 Transmitter is unaffected by the MPCMn 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 ninth bit (RXB8) is used for identifying address and data frames. When the frame type bit (the first
stop or the ninth bit) is '1', the frame contains an address. When the frame type bit is '0', the frame is a
data frame.
The Multi-Processor Communication mode enables several slave MCUs to receive data from a master
MCU. This is done by first decoding an address frame to find out which MCU has been addressed. If a
Atmel ATmega644A [DATASHEET]
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