Zilog Z08470 - Manual (Page 219)

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UM008101-0601 Parallel Input/Output
Figure 13. Daisy-Chain Interrupt Servicing
APPLICATIONS
Extending The Interrupt Daisy-chain
Without external logic, a maximum of four Z80 PIO devices may be daisy-
chained into a priority interrupt structure. This limitation allows the inter-
rupt enable status (IEO) to ripple through the entire daisy-chain between
the beginning of M1 and the beginning of IORQ during an interrupt
acknowledge cycle. The interrupt enable status cannot change during M1,
therefore, the vector address returned to the CPU is assured to be from the
highest priority device that requested an interrupt.
IEI IEO
HI
+
IEI IEO
HI
+
IEI IEO
HI
+
IEI IEO
HI
+
IEI IEO
HI
IEI IEO
HI
IEI IEO
HI
IEI IEO
HI
IEI IEO
HI
IEI IEO
HI
+
IEI IEO
HI
IEI IEO
HI
IEI IEO
LO
IEI IEO
HI
IEI IEO
HI
IEI IEO
HI
IEI IEO
LO
IEI IEO
LO
IEI IEO
LO
IEI IEO
HI
HI
LO
LO
LO
HI
Port 2B Port 1B Port 2A
Under Service
Service Suspended
Service Resumed
Service Complete
5. Second ‘RETI’ instruction issued on completion of Port 2A service routine.
4. Port 1B service routine complete, ‘RETI’ issued, Port 2A service resumed.
3. Port 1B1 interrupts, suspends servicing of Port 2A.
2. Port 2A requests an interrupt and is Acknowledged.
1. Priority interrupt daisy chain before any interrupt occurs.
Service Complete
Under Service
Port 1A
Highest Priority Channel