Multidrop Mode - Freescale Semiconductor MCF54455 Reference Manual

UART Modules
Because the transmitter is inactive, USRn[TXEMP,TXRDY] is inactive and data is sent as it is received.
Received parity is checked but not recalculated for transmission. Character framing is also checked, but
stop bits are sent as they are received. A received break is echoed as received until the next valid start bit
is detected.
32.4.3.2 Local Loopback Mode
Figure 32-22 shows how UnTXD and UnRXD are internally connected in local loopback mode. This
mode is for testing the operation of a UART by sending data to the transmitter and checking data
assembled by the receiver to ensure proper operations.
Features of this local loopback mode are:
• Transmitter and CPU-to-receiver communications continue normally in this mode.
• UnRXD input data is ignored.
• UnTXD is held marking.
• The receiver is clocked by the transmitter clock. The transmitter must be enabled, but the receiver
need not be.
32.4.3.3 Remote Loopback Mode
In remote loopback mode, shown in
bit on the UnTXD output. The local CPU-to-transmitter link is disabled. This mode is useful in testing
receiver and transmitter operation of a remote UART. For this mode, transmitter uses the receiver clock.
Because the receiver is not active, received data cannot be read by the CPU and all status conditions are
inactive. Received parity is not checked and is not recalculated for transmission. Stop bits are sent as they
are received. A received break is echoed as received until next valid start bit is detected.
32.4.4

Multidrop Mode

Setting UMR1n[PM] programs the UART to operate in a wake-up mode for multidrop or multiprocessor
applications. In this mode, a master can transmit an address character followed by a block of data
characters targeted for one of up to 256 slave stations.
32-23
Rx
CPU
Tx
Figure 32-22. Local Loopback
Figure 32-23, the UART automatically transmits received data bit by
Disabled
Rx
CPU
Disabled
Tx
Figure 32-23. Remote Loopback
Disabled
UnRXD Input
Disabled
UnTXD Output
Disabled
UnRXD Input
Disabled UnTXD Output
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