Datasheet

353
32133D–11/2011
UC3D
20.6.5
20.6.6 Test Modes
The internal loopback feature enables on-board diagnostics, and allows the USART to operate
in three different test modes, with reconfigured pin functionality, as shown below.
20.6.6.1 Normal Mode
During normal operation, a receivers RXD pin is connected to a transmitters TXD pin.
Figure 20-20. Normal Mode Configuration
20.6.6.2 Automatic Echo Mode
Automatic echo mode allows bit-by-bit retransmission. When a bit is received on the RXD pin, it
is also sent to the TXD pin, as shown in Figure 20-21. Transmitter configuration has no effect.
Figure 20-21. Automatic Echo Mode Configuration
20.6.6.3 Local Loopback Mode
Local loopback mode connects the output of the transmitter directly to the input of the receiver,
as shown in Figure 20-22. The TXD and RXD pins are not used. The RXD pin has no effect on
the receiver and the TXD pin is continuously driven high, as in idle state.
Figure 20-22. Local Loopback Mode Configuration
20.6.6.4 Remote Loopback Mode
Remote loopback mode connects the RXD pin to the TXD pin, as shown in Figure 20-23. The
transmitter and the receiver are disabled and have no effect. This mode allows bit-by-bit
retransmission.
Receiver
Transmitter
RXD
TXD
Receiver
Transmitter
RXD
TXD
Receiver
Transmitter
RXD
TXD
1