TS68HC901 datasheet(11/42 Pages) STMICROELECTRONICS

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DAISY-CHAINING CMFPs

As an interrupt controller, the TS68HC901 CMFP

will support eight external interrupt sources in addi-

tion to its eight internal interrupt sources. When a

system requires more than eight external interrupt

sources to be placed at the same interrupt level,

sources may be added to the prioritized structure by

daisy-chaining CMFPs. Interrupt sources are priori-

tized internally within each CMFP and the CMFPs

are prioritized by their position in the chain. Unique

vector numbers are provided for each interrupt

sources.

The IEI and IEO signals implement the daisy-chai-

ned interrupt structure. The IEI of the highest priority

CMFP is tied low and the IEO output of this device

is tied to the next highest priority CMFP’s IEI. The

IEI and IEO signals are daisy-chained in this manner

for all CMFPs in the chain, with the lowest priority

CMFP’s IEO left unconnected. A diagram of an in-

terrupt daisy-chain is shown in figure 8.

Daisy-chaining requires that all parts in the chain

have a common IACK. When the common IACK is

asserted during an interrupt acknowledge cycle, all

parts will prioritize interrupts in parallel. When the IEI

signal to a CMFP is asserted, the part may respond

to the IACK cycle if it requires interrupt service.

Otherwise, the part will assert IEO to the next lower

priority device. Thus, priority is passed down the

chain via IEI and IEO until a part which has appen-

ding interrupt is reached. The part with the pending

interrupt passes a vector number to the processor

and does not propagate IEO.

Figure 9a :

Figure 9b :

TS68HC901

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