SAM7X128_14 Datasheet, PDF (316/662 Pages) ATMEL Corporation – ARM-based Flash MCU

30.6.7 Modem Mode
The USART features modem mode, which enables control of the signals: DTR (Data Terminal Ready), DSR (Data Set
Ready), RTS (Request to Send), CTS (Clear to Send), DCD (Data Carrier Detect) and RI (Ring Indicator). While
operating in modem mode, the USART behaves as a DTE (Data Terminal Equipment) as it drives DTR and RTS and can
detect level change on DSR, DCD, CTS and RI.
Setting the USART in modem mode is performed by writing the USART_MODE field in the Mode Register (US_MR) to
the value 0x3. While operating in modem mode the USART behaves as though in asynchronous mode and all the
parameter configurations are available.
Table 30-11 gives the correspondence of the USART signals with modem connection standards.
Table 30-11. Circuit References
USART Pin
V24
TXD
2
RTS
4
DTR
20
RXD
3
CTS
5
DSR
6
DCD
8
RI
22
CCITT
103
105
108.2
104
106
107
109
125
Direction
From terminal to modem
From terminal to modem
From terminal to modem
From modem to terminal
From terminal to modem
From terminal to modem
From terminal to modem
From terminal to modem
The control of the DTR output pin is performed by writing the Control Register (US_CR) with the DTRDIS and DTREN
bits respectively at 1. The disable command forces the corresponding pin to its inactive level, i.e. high. The enable
command forces the corresponding pin to its active level, i.e. low. RTS output pin is automatically controlled in this mode
The level changes are detected on the RI, DSR, DCD and CTS pins. If an input change is detected, the RIIC, DSRIC,
DCDIC and CTSIC bits in the Channel Status Register (US_CSR) are set respectively and can trigger an interrupt. The
status is automatically cleared when US_CSR is read. Furthermore, the CTS automatically disables the transmitter when
it is detected at its inactive state. If a character is being transmitted when the CTS rises, the character transmission is
completed before the transmitter is actually disabled.
30.6.8 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.
30.6.8.1 Normal Mode
Normal mode connects the RXD pin on the receiver input and the transmitter output on the TXD pin.
Figure 30-28. Normal Mode Configuration
Receiver
RXD
Transmitter
TXD
SAM7X Series [DATASHEET]
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