
Intelligent Eight-Channel Communications Controller
—
CD1865
Datasheet
31
5.3.3
Priorities and Fair Share
The CD1865 implements a fair-share mechanism to ensure that all channels receive equal service,
without any
‘
data starvation
’
. Fair share works automatically among the channels in one device and
across multiple devices.
Figure 6 on page 31
shows a fair-share operational block diagram. On each of the three service
request lines, the CD1865 monitors both the internal and external value of the line. (The external
value can differ because, in multiple CD1865 applications, it can be driven by other CD1865s.) At
the end of a service acknowledgment bus cycle, the CD1865 checks the state of both request
values. If they are different, the CD1865 determines that there is another part also driving the
request line, and it does not reassert its own request line until the external request has gone
inactive. This inactive level means every other CD1865 with a pending request is serviced;
therefore, it is now okay to reassert requests without controlling host bandwidth.
5.4
Types of Service Requests
The categories of service requests that a CD1865 can generate are explained below. Each channel
’
s
transmitter, receiver, and modem pins require service from the host occasionally; however, each
category of service request conditions can tolerate different latencies in being serviced. Conditions
for service requests fall into three basic categories:
Data is received from the remote device and needs to be transferred to the host.
Figure 6. Internal Fair-Share Operation
R
S
Q
EXTERNAL REQUEST (I/O PIN)
INTERNAL REQUEST
OK TO ASSERT
TO CD1865
INTERNAL REQUEST
LOGIC
LATCH