Datasheet

386
SAM7S Series [DATASHEET]
6175M–ATARM–26-Oct-12
31.6.7 SAM7S512/256/128/64/321/161 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 31-11 gives the correspondence of the USART signals with modem connection standards.
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.
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 31-30. Normal Mode Configuration
Table 31-11. Circuit References
USART Pin V24 CCITT Direction
TXD 2 103 From terminal to modem
RTS 4 105 From terminal to modem
DTR 20 108.2 From terminal to modem
RXD 3 104 From modem to terminal
CTS 5 106 From terminal to modem
DSR 6 107 From terminal to modem
DCD 8 109 From terminal to modem
RI 22 125 From terminal to modem
Receiver
Transmitter
RXD
TXD