at91rm3400 ATMEL Corporation, at91rm3400 Datasheet - Page 291

• Manufacturer Part Number: at91rm3400
• Description: Atmel Advanced At91 Arm Microcontroller
• Manufacturer: ATMEL Corporation

Modem Mode
Test Modes
Normal Mode
1790A–ATARM–11/03
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 53 gives the correspondence of the USART signals with modem connection standards.
Table 53. Circuit References
The control of the RTS and DTR output pins is performed by witting the Control Register
(US_CR) with the RTSDIS, RTSEN, DTRDIS and DTREN bits respectively at 1. The disable
command forces the corresponding pin to its inactive level, i.e. high. The enable commands
force the corresponding pin to its active level, i.e. low.
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 char-
acter transmission is completed before the transmitter is actually disabled.
The USART can be programmed to operate in three different test modes. The internal loop-
back capability allows on-board diagnostics. In the loopback mode the USART interface pins
are disconnected or not and reconfigured for loopback internally or externally.
As a reminder, the normal mode simply connects the RXD pin on the receiver input and the
transmitter output on the TXD pin.
Figure 116. Normal Mode Configuration
USART pin
TXD
RTS
DTR
RXD
CTS
DSR
DCD
RI
V24
2
4
20
3
5
8
22
6
Transmitter
Receiver
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
RXD
TXD
AT91RM3400
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