ox16pci958 Oxford Electrical Products, ox16pci958 Datasheet - Page 27

• Manufacturer Part Number: ox16pci958
• Description: Octal Uart With Interface
• Manufacturer: Oxford Electrical Products

OXFORD SEMICONDUCTOR LTD.
Modem Status Register (MSR)
MSR allows the state of the modem status
lines CTS#, DSR#, RI#, and DCD# to be read
by the host. Bits 7-4 of this read-only register
reflect the assertion state of the corresponding
input pins and bits 3:0 indicate whether
changes have occurred on these inputs since
MSR was last read.
When the UART is operating under interrupts,
the host can be notified of changes in the
modem status lines by enabling modem status
interrupts (set IER3), in which case a priority-5
interrupt is generated when any of MSR3:0
become set. The interrupt is generated
whether the setting of MSR3:0 is caused by
changes on the modem status lines or by
changes of MCR3:0 in loopback mode.
4.7.
Auto flow control consists of two functional parts: auto-CTS and auto-RTS. These features allow the
flow of serial data to be throttled, preventing data loss due to receive buffer overruns, without relying
on fast interrupt service. RTS# and CTS# are often used for flow control, but if the host has to perform
that function then delays in servicing interrupts can mean that the flow control is not quick enough,
and data can be lost. When a pair of UARTs are connected that both have auto flow control enabled,
then loss-free data transfer is possible whatever the interrupt latencies of the host systems.
The RTS# output of the receiving UART must be connected to the CTS# input of the transmitting
UART, as shown below. Usually a full-duplex link will be used and so the diagram below will be only
half the system, with both UARTs having a transmitting and a receiving side and being connected
symmetrically.
DS-0022 Nov 05
Auto Flow Control
Transmit
FIFO
Transmitting UART
Figure 1 Auto Flow Control (Auto-RTS & Auto-CTS) Example
Parallel to
Control
Serial
Flow
SOUT
CTS#
External—Free Release
SIN
RTS#
Bit
7
6
5
4
3
2
1
0
Description
Set when the DCD# input is low.
In loopback mode (MCR4 is set), MSR7
reflects the value last written to MCR3
Set when the RI# input is low.
In loopback mode, MSR6 reflects the value
last written to MCR2
Set when the DSR# input is low.
In loopback mode, MSR5 reflects the value
last written to MCR0
Set when the CTS# input is low.
In loopback mode, MSR4 reflects the value
last written to MCR1
1—DCD# input has changed state since
the last time the MSR was read
1—RI# input has changed from a low to a
high state since the last time the MSR was
read. High-to-low transitions on RI# do not
affect TERI
1—DSR# input has changed state since
the last time the MSR was read
1—CTS# input has changed state since
the last time the MSR was read
Serial to
Parallel
Control
Flow
Receiving UART
OX16PCI958 DATA SHEET
Receive
FIFO
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