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8210C–AVR–09/11
Atmel AVR XMEGA D
• Interrupt timing is not compatible
• Pin control differs due to the master-only operation of the USART in SPI master mode
A comparison of the USART in master SPI mode and the SPI pins is shown
.
18.12 Multiprocessor Communication Mode
The multiprocessor communication mode effectively reduces the number of incoming frames
that have to be handled by the receiver in a system with multiple microcontrollers communicat-
ing via the same serial bus. In this mode, a dedicated bit in the frames is used to indicate
whether the frame is an address or data frame type.
If the receiver is set up to receive frames that contain five to eight data bits, the first stop bit is
used to indicate the frame type. If the receiver is set up for frames with nine data bits, the ninth
bit is used. When the frame type bit is one, the frame contains an address. When the frame type
bit is zero, the frame is a data frame. If 5-bit to 8-bit character frames are used, the transmitter
must be set to use two stop bits, since the first stop bit is used for indicating the frame type.
If a particular slave MCU has been addressed, it will receive the following data frames as usual,
while the other slave MCUs will ignore the frames until another address frame is received.
18.12.1
Using Multiprocessor Communication Mode
The following procedure should be used to exchange data in multiprocessor communication
mode (MPCM):
1.
All slave MCUs are in multiprocessor communication mode.
2.
The master MCU sends an address frame, and all slaves receive and read this frame.
3.
Each slave MCU determines if it has been selected.
4.
The addressed MCU will disable MPCM and receive all data frames. The other slave
MCUs will ignore the data frames.
5.
When the addressed MCU has received the last data frame, it must enable MPCM
again and wait for a new address frame from the master.
The process then repeats from step 2.
Using any of the 5-bit to 8-bit character frame formats is impractical, as the receiver must
change between using n and n+1 character frame formats. This makes full-duplex operation dif-
ficult, since the transmitter and receiver must use the same character size setting.
18.13 IRCOM Mode of Operation
IRCOM mode can be enabled to use the IRCOM module with the USART. This enables IrDA 1.4
compliant modulation and demodulation for baud rates up to 115.2kbps. When IRCOM mode is
enabled, double speed mode cannot be used for the USART.
Table 18-5.
Comparison of USART in master SPI mode and SPI pins.
USART
SPI
Comment
TxD
MOSI
Master out only
RxD
MISO
Master in only
XCK
SCK
Functionally identical
N/A
SS
Not supported by USART in master SPI mode