ATmega128 datasheet(188/386 Pages) ATMEL

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2467S–AVR–07/09

ATmega128

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.

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:

All slave MCUs are in Multi-processor Communication mode (MPCM in UCSRA is set).

The master MCU sends an address frame, and all slaves receive and read this frame. In

the slave MCUs, the RXC flag in UCSRA will be set as normal.

Each slave MCU reads the UDR Register and determines if it has been selected. If so, it

clears the MPCM bit in UCSRA, otherwise it waits for the next address byte and keeps

the MPCM setting.

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.

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 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 opera-

tion 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.

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