CD1865 datasheet(27/150 Pages) INTEL

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Intelligent Eight-Channel Communications Controller — CD1865

Datasheet

If AutoPri is not being used, the CD1865 needs to be informed which one of the three possible

pending requests the host wants to acknowledge. There are two different ways CD1865 can be

informed of this — hardware and software.

The hardware method is based on the value in the address bus. The CD1865 determines the type of

request being acknowledged by the value placed in the address bus during the acknowledge cycle.

This is the method used by Motorola-family processors. The host places the level of interrupt

being serviced on the low-order address bits during an interrupt acknowledgment cycle. When the

host performs a Service Acknowledge cycle, the CD1865 compares the value on the address bus

with the three unique values stored in three internal registers — the . These values are set by the

user at system initialization. A match occurs on only one of these registers, and this informs the

CD1865 of the type of request being acknowledged.

In most circumstances the address bus should not have a value that does not match one of the three

values during an acknowledgment cycle. This causes the CD1865 to not recognize that any bus

cycle is occurring, and it does not assert DTACK*, or terminate the cycle, or take any other action.

Doing this does not affect the CD1865, but the system must have some other provision to terminate

the bus cycle. If, for example, the CD1865 shares an interrupt level with another device, different

values on the address bus should be used to control responses to an acknowledgment, but the bus

cycle should terminate in a usable way.

Service acknowledgments can also be performed by software. The host simply reads one of three

Request Acknowledge registers, and the CD1865 performs as if a hardware service acknowledge

cycle is executed.

Regardless of the method of acknowledgment used, within the CD1865, each service request state

machine makes the following determination: if it has an internal service request pending, and there

is a service acknowledge of the same type, it asserts its internal-acknowledge-accepted signal back

to the Service Request Controller logic, negates the Service Request Output pin, and holds its

acknowledge-out daisy chain in a negated state. It also drives the value in the Global Vector

The GVR value placed on the bus during the Service Acknowledge cycle serves two purposes. The

least-significant three bits of GVR indicate which of the four types of service requests are

occurring. The upper-five bits are user-defined and serve to identify, in daisy-chained CD1865

systems, which of the multiple CD1865s is active.

If the service request state machine does not have a service request pending, and there is a software

acknowledgment or address bus match, it passes the service acknowledgment down the chain by

asserting ACKOUT*. If there is no match, the state machine remains idle.

If a service request is pending and the Receive Service Request is to be handled, the CD1865 is

notified because the three have different values in them; therefore, only one match (receive

service, in this case) occurred. The internal grant from the service request state machine causes the

receive service type code and active channel number (previously stored at the time the request is

posted by the CD1865 processor) to be pushed onto the service request stack. This automatically

causes the FIFO pointers to be set up for the active channel, with no host intervention.

The host, at this point, has all the information needed to handle the service request. It determines

the exact type of service being requested (Transmit, Receive Good Data, Receive Exception, or

Modem signal change) and which of the multiple CD1865s is requesting service. It gets the

channel number by reading the Global Channel register (GCR) and then proceeds to service the

request. At the completion of the service, the host performs a dummy write to the CD1865 End Of

CD1865 Datasheet(PDF) 27 Page - Intel Corporation

CD1865 Datasheet(HTML) 27 Page - Intel Corporation

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