CD1865 datasheet(80/150 Pages) INTEL

80 / 150 page

CD1865 — Intelligent Eight-Channel Communications Controller

Datasheet

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’.

7.4.1

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.

7.4.2

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.

7.5

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

CD1865 Datasheet(PDF) 80 Page - Intel Corporation

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