CD1865 datasheet(29/150 Pages) INTEL

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Intelligent Eight-Channel Communications Controller — CD1865

Datasheet

NOTE: The (†) denotes the point at which, if there is no match, the CD1865 determines not to pass the ACK

down the daisy chain. It does this for two reasons: first, it is unacceptable to have the ACKOUT* ‘glitch’

low; and second, the state machine should be as fast as possible. When the state machine senses an

ACKIN* and match is not valid, it cannot conclude that it should assert ACKOUT*; the ACKIN* may be

for one of the other two service requests levels. It could wait for the results of the other two MATCH

comparators; however, this complicates, and therefore slows down, the response of the state machine.

The reason this complication causes delay is (to implement the logical function ‘assert ACKOUT* if no

match’) it must determine how long to wait before declaring a no-match condition. To implement this

delay function, a synchronous state machine is required, which at a 15-MHz clock, means a delay of

several hundred nanoseconds from ACKIN* to ACKOUT*, instead of the 65 ns currently specified.

Table 3.

State Machine Logic

State Name

Output Condition

Comments

IDLE

IF (INTERNAL_REQUEST = 1)

ELSE IF (ACKIN* = 1 & MATCH =1)

ELSE

all outputs inactive

GoTo REQ_ACTIVE

GoTo PASS_

ACK

Stay at IDLE

; normal ‘resting’ state

; pass this acknowledge

; wait here

REQ_ACTIVE

IF (ACKIN* = 1 & MATCH =1)

IF (ACKIN* = 1 & MATCH =0)

ELSE

GoTo KEEP_ACK

Stay at REQ_ACTIVE

request asserted

; keep this acknowledge

; wait here, ACK is for some other level (†)

; wait here

PASS_ACK

IF (ACKIN* = 0)

ELSE

GoTo IDLE

Stay at PASS_ACK

ACKOUT* asserted

; return when ACKIN* is gone

; wait here while ACKIN* active

KEEP_ACK

IF (ACKIN* = 0)

ELSE

GoTo IDLE

Stay at KEEP_ACK

INTERNAL_GRANT asserted

; return when ACKIN* is gone

; wait here while ACKIN* active

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