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Functional Description
1626
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Universal Asynchronous Receiver/Transmitter (UART)
•
UnCTS: clear to send
•
UnDSR: data set ready
•
UnDCD: data carrier detect
•
UnRI: ring indicator
•
UnRTS: request to send
•
UnDTR: data terminal ready
When used as a DCE, the modem flow control and status signals are defined as:
•
UnCTS: request to send
•
UnDSR: data terminal ready
•
UnRTS: clear to send
•
UnDTR: 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 GPIO signal and providing software
support.
26.3.6.2 Flow Control
Flow control can be accomplished by either hardware or software. The following sections describe the
different methods.
26.3.6.2.1 Hardware Flow Control (RTS and 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 (active low). The UnRTS output signal indicates the state of the receive FIFO. UnCTS remains
asserted (active low) 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 26-1. Flow Control Mode
CTSEN
RTSEN
Description
1
1
RTS and CTS flow control enabled
1
0
Only CTS flow control enabled
0
1
Only RTS flow control enabled
0
0
Both RTS and CTS flow control disabled
When RTSEN is 1, software cannot modify the UnRTS output value through the UARTCTL register
request-to-send (RTS) bit, and the status of the RTS bit should be ignored.
26.3.6.2.2 Software Flow Control (Modem Status Interrupts)
Software flow control between two devices is accomplished by using interrupts to indicate the status of the
UART. Interrupts may be generated for the UnDSR, UnDCD, UnCTS, and UnRI signals using bits 3:0 of
the UARTIM register, respectively. The raw and masked interrupt status may be checked using the
UARTRIS and UARTMIS register. These interrupts may be cleared using the UARTICR register.