CD1865 datasheet(41/150 Pages) INTEL

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Intelligent Eight-Channel Communications Controller — CD1865

Datasheet

There is a difference between the CD1865 and conventional dumb UARTs that makes the CD1865

more efficient even when operating in a polled environment. With a dumb UART, the host polls

each channel in turn to determine whether it has any data. With the CD1865, the host polls the

CD1865s as a group for whether it has data. If it does, the CD1865s indicates the channel, rather

than the host testing each channel in turn. In fact, it is not possible for the host to dictate which

channel is serviced; the CD1865 determines this order. This minimizes both the number of polling

steps required and the amount of time each needs. This also ensures fair, balanced service of all

channels.

There are several ways that a host system can poll the CD1865. Each method has certain

advantages. The most direct method is to read the Service Request Status register (SRSR). This

modem signal change, on the CD1865 being read. There are three more bits that provide the same

information for all CD1865s in the system — these three bits reflect the state of the wire-OR’ed

external request lines. Thus a single read operation can determine if there is any activity.

Figure 11. Simple Software Polled Interface Example

CD1865 #2

CD1865 #1

MICROPROCESSOR

A1–A3

A4–A7

D0–D7

A0–A2

A3–A6

A0–A2

A3–A6

D0–D7

RREQ*

TREQ*

MREQ*

RREQ*

TREQ*

MREQ*

ACKIN* ACKOUT*

ACKOUT* ACKIN*

CD1865 Datasheet(PDF) 41 Page - Intel Corporation