Datasheet

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.
21.10.1. Using MPCMn
For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZ1=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 UCSRnA is set).
2. The Master MCU sends an address frame, and all slaves receive and read this frame. In the Slave
MCUs, the RXC Flag in UCSRnA will be set as normal.
3. Each Slave MCU reads the UDRn Register and determines if it has been selected. If so, it clears
the MPCM bit in UCSRnA, otherwise it waits for the next address byte and keeps the MPCM
setting.
4. The addressed MCU will receive all data frames until a new address frame is received. The other
Slave MCUs, which still have the MPCM bit set, will ignore the data frames.
5. When the last data frame is received by the addressed MCU, the addressed MCU sets the MPCM
bit and waits for a new address frame from master. The process then repeats from step 2.
Using any of the 5- to 8-bit character frame formats is possible, but impractical since the Receiver must
change between using n and n+1 character frame formats. This makes full-duplex operation difficult since
the Transmitter and Receiver uses the same character size setting. If 5- to 8-bit character frames are
used, the Transmitter must be set to use two stop bit (USBS = 1) since the first stop bit is used for
indicating the frame type.
Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCM bit. The MPCM bit
shares the same I/O location as the TXC Flag and this might accidentally be cleared when using SBI or
CBI instructions.
21.11. Examples of Baud Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous
operation can be generated by using the UBRRn settings as listed in the table below.
UBRRn values which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold
in the table. Higher error ratings are acceptable, but the Receiver will have less noise resistance when the
error ratings are high, especially for large serial frames (see also section Asynchronous Operational
Range). The error values are calculated using the following equation:
 % =
BaudRate
Closest Match
BaudRate
 1
2
100 %
Table 21-4. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud
Rate
[bps]
f
osc
= 1.0000MHz f
osc
= 1.8432MHz f
osc
= 2.0000MHz
U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%
4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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