ATmega16 datasheet(154/315 Pages) ATMEL

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154

ATmega16(L)

2466E–AVR–10/02

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 Trans-

mitter 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.

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 Communi-

cation 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 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.

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