Datasheet

Table 25-3 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 (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.
25.9. Multi-Processor Communication Mode
Setting the Multi-processor Communication mode (MPCM) bit in UCSRA 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 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 nine 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 one, the frame contains an address. When the frame type bit is zero 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
particular Slave MCU has been addressed, it will receive the following data frames as normal, while the
other Slave MCUs will ignore the received frames until another address frame is received.
25.9.1. Using MPCM
For an MCU to act as a Master MCU, it can use a 9-bit character frame format (UCSZ = 7). The ninth bit
(TXB8) must be set when an address frame (TXB8 = 1) or cleared when a data frame (TXB = 0) is being
transmitted. The Slave MCUs must in this case be set to use a 9-bit character frame format.
The following procedure should be used to exchange data in Multi-Processor Communication Mode:
1. All Slave MCUs are in Multi-processor Communication mode (MPCM in UCSRA is set).
Atmel ATmega64A [DATASHEET]
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