CD1865 Datasheet, PDF (80/150 Pages) Intel Corporation – Intelligent Eight-Channel Communications Controller

CD1865 — Intelligent Eight-Channel Communications Controller
7.4.1
7.4.2
7.5
When the CD1865 is programmed to detect level changes and generate service requests when level
changes occur, it does so in firmware by reading the pins and comparing to a previously stored
value. This function is performed in the main timing loop of the firmware; the maximum time
required to detect a level change under worst-case conditions is approximately 2 ms. When the
CD1865 is performing this function, the modem pins are periodically sampled rather than
continuously monitored; as such they have very little sensitivity to noise, which is desirable in data
communication applications. However, in extremely noisy applications, re-read a modem line
which has caused a Modem Signal Change Service Request to verify that it has indeed changed and
is not merely malfunctioning. This eliminates even the slight possibility of a noise pulse causing
erratic operation.
When the CD1865 is monitoring modem pins to control transmit or receive functions, it does not
rely on the previously stored value, but checks the pins at the appropriate time. Thus, there is very
little delay in this response. For example, before deciding to transmit another character, it examines
the CTS* pin at that time. (The CD1865 makes this decision when moving characters from the
FIFO to the Holding register, not from the Holding register to the Shift register.) Refer to
Section 7.3 on page 72 for flow-control details.
Note that the logical sense of the modem bits is inverted; for example, writing a ‘1’ to the MSVR
causes the output pin to go to nominal zero volts. Likewise, a low-voltage input is sensed as a ‘1’.
Generating Service Requests with Modem Pins
The CD1865 can generate service requests when any one of the input pins changes state. Either or
both edges may be detected by setting bits in the two Modem Change Option registers (MCOR1
and MCOR2). For each pin, the user can individually enable on-to-off or off-to-on transition
detection of the inputs. When the CD1865 detects such a transition, it sets the corresponding bit in
the Modem Change register. If the corresponding bit in the channel’s set, the CD1865 asserts its *
output. The user must clear the Modem Change register during the service request service routine
before writing to the .
The CD1865 performs this task by reading the modem input signals and comparing the current
value with the value read in the last pass through the outer scanning loop. Because this is the
lowest-priority event in the CD1865 scanning loop, changes may not be detected unless they are
several hundred microseconds long. Modem Input pins can be used for purposes such as detecting
the closing of a switch. However, the relatively slow speed of response should be taken into
account when using Modem Input pins for this purpose. The CD1865 does not latch the Modem
input signals.
Using Modem Pins as General-Purpose I/O
Since the modem pins can be directly accessed by the host, they can be used as general-purpose I/O
pins if they are not needed for flow control or modem interfacing. Simply read from and write to
them as any I/O port.
Testing the CD1865 — Loopback Tests
The CD1865 performs a basic internal self-test whenever it is reset. This test provides a reasonable
degree of confidence that the CD1865 is functioning satisfactorily. There are two additional tests
that can be performed by the user to further ensure complete functionality. These two test modes
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Datasheet