16.3.6 Modem Handshake Support
This section describes how to configure and use the modem flow control and status signals for a
UART when connected as a DTE (data terminal equipment) or as a DCE (data communications
equipment). In general, a modem is a DCE and a computing device that connects to a modem is
the DTE.
16.3.6.1 Signaling
The status signals provided by a UART differ based on whether the UART is used as a DTE or
DCE. When used as a DTE, the modem flow control and status signals are defined as:
■ UnCTS is Clear To Send
■ UnDSR is Data Set Ready
■ UnDCD is Data Carrier Detect
■ UnRI is Ring Indicator
■ UnRTS is Request To Send
■ UnDTR is Data Terminal Ready
When used as a DCE, the modem flow control and status signals are defined as:
■ UnCTS is Request To Send
■ UnDSR is Data Terminal Ready
■ UnRTS is Clear To Send
■ UnDTR is Data Set Ready
Note that the support for DCE functions Data Carrier Detect and Ring Indicator are not provided. If
these signals are required, their function can be emulated by using a general-purpose I/O signal
and providing software support.
16.3.6.2 Flow Control
Flow control can be accomplished by either hardware or software. The following sections describe
the different methods.
Hardware Flow Control (RTS/CTS)
Hardware flow control between two devices is accomplished by connecting the UnRTS output to the
Clear-To-Send input on the receiving device, and connecting the Request-To-Send output on the
receiving device to the UnCTS input.
The UnCTS input controls the transmitter. The transmitter may only transmit data when the UnCTS
input is asserted. The UnRTS output signal indicates the state of the receive FIFO. UnCTS remains
asserted until the preprogrammed watermark level is reached, indicating that the Receive FIFO has
no space to store additional characters.
The UARTCTL register bits 15 (CTSEN) and 14 (RTSEN) specify the flow control mode as shown in
Table 16-2 on page 1169.
June 18, 20141168
Texas Instruments-Production Data
Universal Asynchronous Receivers/Transmitters (UARTs)
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