CD1865 Datasheet, PDF (35/150 Pages) Intel Corporation – Intelligent Eight-Channel Communications Controller

Intelligent Eight-Channel Communications Controller — CD1865
5.4.2
5.4.3
5.4.3.1
5.5
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).
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.
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.
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:
Datasheet
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