Motorola ColdFire MCF5281 - Manual (Page 443)

UART Modules
Freescale Semiconductor 23-23
23.4.3.1 Automatic Echo Mode
In automatic echo mode, shown in Figure 23-21 , the UART automatically resends received data bit by bit.
The local CPU-to-receiver communication continues normally, but the CPU-to-transmitter link is
disabled. In this mode, received data is clocked on the receiver clock and re-sent on UTXDn. The receiver
must be enabled, but the transmitter need not be.
Figure 23-21. Automatic Echo
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.
23.4.3.2 Local Loopback Mode
Figure 23-22 shows how UTXDn and URXDn 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.
Figure 23-22. Local Loopback
Features of this local loopback mode are:
• Transmitter and CPU-to-receiver communications continue normally in this mode.
• URXDn input data is ignored.
• UTXDn is held marking.
• The receiver is clocked by the transmitter clock. The transmitter must be enabled, but the receiver
need not be.
23.4.3.3 Remote Loopback Mode
In remote loopback mode, shown in Figure 23-23 , the UART automatically transmits received data bit by
bit on the UTXDn 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.
Disabled Disabled
Tx
Rx
CPU
URXDn Input URXDn Input
UTXDn Output
CPU
Disabled
Disabled
Tx
Rx URXDn Input
UTXDn Output
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3