CD1865 datasheet(35/150 Pages) INTEL

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Intelligent Eight-Channel Communications Controller — CD1865

Datasheet

5.4.2

Transmit Service Requests

Each transmitter contains 8 bytes of Transmit FIFO in addition to the Transmit Holding register

and the Transmit Shift register. As data is being transmitted, the FIFO status is being monitored by

the CD1865. A service request is invoked for one of the following conditions:

• Transmit FIFO Empty — When the Transmit FIFO is empty, there is still one character in the

Transmit Holding register and one character in the Transmit Shift register. The host has two

character times to respond to this request without causing a gap in the Transmit Data Stream.

• Transmitter Empty — The Transmit FIFO, Transmit Holding register, and the Transmit Shift

registers are now empty. This signifies that all characters written to the FIFO are completely

transmitted.

The host can select which one of these causes a Transmit Service Request, and it is used by

programming the options in the Service Request Enable register (SRER).

Data must not be put into the Transmit FIFO at any time other than when the CD1865 is in a

Transmit Service Request context for that channel. During a transmit service, characters (up to

eight) are placed into the FIFO by the Transmit Data register (TDR).

5.4.3

Modem Signal Change Service Requests

The CD1865 can be programmed to assert a service request when a channel’s modem input signals

has changed states. The change-detect options are programmed in the Modem Change Option

registers. Individual modem pin service requests are enabled by setting the corresponding bits in

the Service Request Enable register.

The host must read the Modem Change register during a modem change service to determine

which modem signal changes were detected. This is indicated by a ‘1’ in the appropriate bit

location. The Modem Change register must be reset to a ‘0’ by the host before exiting the service

request because the CD1865 does not do this. Refer to Section 7.4 for more details.

5.4.3.1

Using Modem Pins as Input/Output

The pins labelled as modem pins are general-purpose I/O pins that can be controlled by either the

CD1865 processor or the host system. There is no direct, hardwired connection from any modem

pin directly to a transmitter or a receiver. This means that these pins can be used for general-

purpose I/O if they are not needed for modem-control purposes. See Section 7.4 for more details.

5.5

Implementing Service Requests

The CD1865 is designed to easily interface with any processor, yet be efficient and flexible enough

to provide maximum throughput. The CD1865 generates service requests and waits for

acknowledgments of these from the host. However, service requests can be implemented in either

hardware or software; likewise, acknowledgments can be affected either way to offer maximum

advantages to the system designer and programmer. This interfacing can be grouped as various

steps.

Service requests must be ‘noticed’ by the host system before they can be acted on, and this can be

done the following three ways:

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