Datasheet

Chapter 14. UART Modules 14-25
Operation
available; therefore, overrun errors can be prevented by connecting RTS to the CTS input
of the transmitting device.
NOTE:
The receiver can still read characters in the FIFO stack if the
receiver is disabled. If the receiver is reset, the FIFO stack, R
TS
control, all receiver status bits, and interrupt requests are reset.
No more characters are received until the receiver is reenabled.
14.5.3 Looping Modes
The UART can be configured to operate in various looping modes as shown in Figure 14-22
on page 14-23. These modes are useful for local and remote system diagnostic functions.
The modes are described in the following paragraphs and in Section 14.3, “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.
14.5.3.1 Automatic Echo Mode
In automatic echo mode, shown in Figure 14-23, 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 resent on TxD. The receiver must be enabled, but the transmitter need not be.
Figure 14-23. Automatic Echo
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.
14.5.3.2 Local Loop-Back Mode
Figure 14-24 shows how TxD and RxD 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.
CPU
Disabled Disabled
RxD Input
TxD Input
Tx
Rx
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