Datasheet

SAM9G20
DS60001516A-page 472  2017 Microchip Technology Inc.
31.6.8 Test Modes
The USART can be programmed to operate in three different test modes. The internal loopback capability allows on-board diagnostics. In
the loopback mode the USART interface pins are disconnected or not and reconfigured for loopback internally or externally.
31.6.8.1 Normal Mode
Normal mode connects the RXD pin on the receiver input and the transmitter output on the TXD pin.
Figure 31-38: Normal Mode Configuration
31.6.8.2 Automatic Echo Mode
Automatic echo mode allows bit-by-bit retransmission. When a bit is received on the RXD pin, it is sent to the TXD pin, as shown in
Figure 31-39. Programming the transmitter has no effect on the TXD pin. The RXD pin is still connected to the receiver input, thus the
receiver remains active.
Figure 31-39: Automatic Echo Mode Configuration
31.6.8.3 Local Loopback Mode
Local loopback mode connects the output of the transmitter directly to the input of the receiver, as shown in Figure 31-40. 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 31-40: Local Loopback Mode Configuration
31.6.8.4 Remote Loopback Mode
Remote loopback mode directly connects the RXD pin to the TXD pin, as shown in Figure 31-41. 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