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UART Modules
MCF5213 ColdFire® Integrated Microcontroller Reference Manual, Rev. 3
21-22
Freescale Semiconductor
21.4.3
Looping Modes
The UART can be configured to operate in various looping modes, as shown in
are useful for local and remote system diagnostic functions. The modes are described in the following
paragraphs and in
Section 21.3, “Memory Map/Register Definition.”
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.
21.4.3.1
Automatic Echo Mode
In automatic echo mode, shown in
, 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 U
n
TXD. The receiver
must be enabled, but the transmitter need not be.
Figure 21-21. Automatic Echo
Because the transmitter is inactive, USR
n
[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.
21.4.3.2
Local Loop-Back Mode
shows how U
n
TXD and U
n
RXD are internally connected in local loop-back 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 21-22. Local Loop-Back
Features of this local loop-back mode are:
•
Transmitter and CPU-to-receiver communications continue normally in this mode.
•
U
n
RXD input data is ignored.
•
U
n
TXD is held marking.
•
The receiver is clocked by the transmitter clock. The transmitter must be enabled, but the receiver
need not be.
Disabled
Disabled
U
n
RXD Input
U
n
TXD Output
Tx
Rx
CPU
CPU
Disabled
Disabled
U
n
RXD Input
U
n
TXD Output
Tx
Rx