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13.2.3 Asynchronous Communication Formats
When two devices communicate asynchronously, the idle-line format is used
for the protocol. When three or more devices communicate, the USART
supports the idle-line and address-bit multiprocessor communication formats.
Idle-Line Multiprocessor Format
When MM = 0, the idle-line multiprocessor format is selected. Blocks of data
are separated by an idle time on the transmit or receive lines as shown in
Figure 13−3. An idle receive line is detected when 10 or more continuous ones
(marks) are received after the first stop bit of a character. When two stop bits
are used for the idle line the second stop bit is counted as the first mark bit of
the idle period.
The first character received after an idle period is an address character. The
RXWAKE bit is used as an address tag for each block of characters. In the
idle-line multiprocessor format, this bit is set when a received character is an
address and is transferred to UxRXBUF.
Figure 13−3. Idle
Line Format
-
UTXDx/URXDx
UTXDx/URXDx Expanded
UTXDx/URXDx
ST
First Character Within Block
Is Address. It Follows Idle
Period of 10 Bits or More
Blocks of
Characters
Address
SP ST
Character Within Block
USART Peripheral Interface, UART Mode
USART Operation: UART Mode
Idle Periods of 10 Bits or More
Data
SP
ST
Character Within Block
Idle Period Less Than 10 Bits
Data
SP
13-5
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