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23.5.3 Looping Modes
The UART can be configured to operate in various looping modes as shown in
Figure 23-21. These modes are useful for local and remote system diagnostic functions.
The modes are described in the following paragraphs and in Section 23.3, "Register
Descriptions."
The UART's transmitter and receiver should be disabled when switching between modes.
The selected mode is activated immediately upon mode selection, regardless of whether a
character is being received or transmitted.
23.5.3.1 Automatic Echo Mode
In automatic echo mode, shown in Figure 23-22, 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 UTXD. The receiver must be enabled, but the transmitter need not be.
Because the transmitter is inactive, USRn[TxEMP,TxRDY] are inactive and data is sent as
it is received. Received parity is checked but is 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.
23.5.3.2 Local Loop-Back Mode
Figure 23-23 shows how UTXDn and URXDn are internally connected in local loop-back
mode. This mode is for testing the operation of a local UART module channel by sending
data to the transmitter and checking data assembled by the receiver to ensure proper
operations.
Features of this local loop-back mode are as follows:
• Transmitter and CPU-to-receiver communications continue normally in this mode.
• URXDn input data is ignored
MOTOROLA
Rx
CPU
Disabled
Tx
Figure 23-22. Automatic Echo
Rx
CPU
Tx
Figure 23-23. Local Loop-Back
Chapter 23. UART Modules
URXDn Input
Disabled
UTXDn Output
Disabled
URXDn Input
Disabled
UTXDn Output
Operation
23-25
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