CD1865 Datasheet, PDF (41/150 Pages) Intel Corporation – Intelligent Eight-Channel Communications Controller

Intelligent Eight-Channel Communications Controller — CD1865
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
register contains three bits that indicate whether there is a request pending for receive, transmit, or
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
RREQ*
TREQ*
MREQ*
ACKOUT* ACKIN*
CD1865 #2
D0–D7
A3–A6
A0–A2
MICROPROCESSOR
A4–A7
A1–A3
D0–D7
RREQ*
TREQ*
MREQ*
ACKIN* ACKOUT*
CD1865 #1
D0–D7
A3–A6
A0–A2
Datasheet
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