User's Manual

© SENSITECH INC. CONFIDENTIAL – CONTROLLED DOCUMENT Page 37 of 83
A Carrier Corp. Company
PART NUMBER T82002163
REV A
nDSR Input
Data Set Ready—When low, indicates that the modem or data set is ready to establish a
communications link with a UART. The nDSR signal is a modem-status input. Its
condition can be tested by reading MSR[DSR], which is the complement of nDSR.
MSR[DDSR] indicates whether the nDSR input has changed state since MSR was last
read. This signal is present only on the FFUART. When MSR[DSR] changes state, an
interrupt is generated if the modem-status interrupt is enabled.
nDCD Input
Data Carrier Detect—When low, indicates that the data carrier has been detected by the
modem or data set. The nDCD signal is a modem-status input. Its condition can be tested
by reading MSR[DCD], which is the complement of the nDCD signal. MSR[DDCD]
indicates whether the nDCD input has changed state since the previous reading of MSR.
nDCD has no effect on the receiver. This signal is present only on the FFUART. When
the DCD bit changes state and the modem-status interrupt is enabled, an interrupt is
generated.
Table 10-1. UARTs I/O Signal Descriptions (Sheet 2 of 2)
Name Type Description
nRI Input
Ring Indicator—When low, indicates that the modem or data set has received a telephone
ringing signal. The nRI signal is a modem-status input. Its condition can be tested by reading
MSR[RI], which is the complement of the nRI signal. MSR[TERI] (trailing-edge-of-ring
indicator) indicates whether the nRI input has changed from low to high since MSR was last
read. This signal is present only on the FFUART. When the RI bit of the MSR changes from
a high to low state and the modem-status interrupt is enabled, an interrupt is generated.
nDTR Output
Data Terminal Ready—When low, signals the modem or the data set that the UART is
ready to establish a communications link. To assert the nDTR output (active low), set
MSR[DTR], which is the complement of the output signal. A reset operation deasserts this
signal (high). Loopback-mode operation holds nDTR deasserted. This signal is present only
on the FFUART.
nRTS Output
Request to Send—When low, signals the modem or the data set that the UART is ready to
exchange data. To assert the nRTS output (active low), set MSR[RTS], which is the
complement of the output signal. A reset operation deasserts this signal (high). Loopback-
mode operation holds nRTS deasserted. This signal is used by the FFUART and BTUART.
Non-Auto-Flow Mode—To assert the nRTS output (active low), set MSR[RTS]. Auto-Flow
Mode—nRTS is automatically asserted by the auto-flow circuitry when the receive buffer
exceeds its programmed trigger threshold. It is deasserted when enough bytes are removed
from the buffer to lower the data level back to the trigger threshold.
Operation
Figure 10-1 shows the format of a UART data frame.
Figure 10-1. Example UART Data Frame