Atmel SAM4S Series, Preliminary Data Sheet

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766
11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
reception but only a low level. However, this low level must be present on the slave select line
(NSS) at least 1 Tbit before the first serial clock cycle corresponding to the MSB bit.
34.7.8.7 Receiver Timeout
Because the receiver baudrate clock is active only during data transfers in SPI Mode, a receiver
timeout is impossible in this mode, whatever the Time-out value is (field TO) in the Time-out
Register (US_RTOR).
34.7.9 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 dis-
connected or not and reconfigured for loopback internally or externally.
34.7.9.1 Normal Mode
Normal mode connects the RXD pin on the receiver input and the transmitter output on the
TXD pin.
Figure 34-40. Normal Mode Configuration
34.7.9.2 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 34-41 . 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 34-41. Automatic Echo Mode Configuration
34.7.9.3 Local Loopback Mode
Local loopback mode connects the output of the transmitter directly to the input of the receiver,
as shown in
Figure 34-42 . 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.
Receiver
Transmitter
RXD
TXD
Receiver
Transmitter
RXD
TXD