MC68HC901 Datasheet, PDF (26/73 Pages) Motorola, Inc – Multi-Function Peripheral

Interrupt Structure
V7-V4 — Vector
These upper four bits of the vector register are written by the user. These bits become the
most-significant four bits of the interrupt vector number.
S — In-Service Register Enable
1 = Software end-of-interrupt mode and in-service register bits enabled.
0 = Automatic end-of-interrupt mode and in-service register bits forced low.
Bits 2-0 — *Unused
Unused bits read as zero.
4.2 DAISY-CHAINING MFPs
As an interrupt controller, the MFP will support eight external interrupt sources in addition
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 MFPs. Interrupt sources are prioritized internally
within each MFP, and the MFPs are prioritized by their position in the chain. Unique vector
numbers are provided for each interrupt source.
The IEI and IEO signals implement the daisy-chained structure. The IEI of the highest
priority MFP is tied low and the IEO output of this device is tied to the next highest priority
MFP IEI. The IEI and IEO signals are daisy-chained in this manner for all the MFPs in the
chain with the lowest priority MFP IEO left unconnected. Figure 4-1 shows a diagram of the
interrupt daisy-chain.
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 an MFP 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 a
pending interrupt is reached. The part with the pending interrupt passes a vector number to
the processor and does not propagate IEO.
HIGHEST
PRIORITY
MC68HC901
MC68HC901
LOWEST
PRIORITY
MC68HC901
IEI IEO
IEI IEO
IEI
IACK
IACK
IACK
IACK
Figure 4-1. Daisy-Chained Interrupt Structure
FIGURE - DAISY-CHAINED INTERRUPT STRUCTURE
MC68HC901UM / AD
MOTOROLA
MC68HC901 USER’S MANUAL
4-3