Z80 Datasheet, PDF (329/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

Z80 CPU Peripherals
User Manual
309
executed or a RETI command is written to the Z80 SIO, resetting the
interrupt-under-service latch of the Channel A interrupt. At this time, the
service routine for Channel B is resumed. When the service routine is
completed, another RETI instruction is executed to complete the interrupt
service.
Channel A
Receiver
Channel A
Transmitter
Channel A
External Status
Channel B
Receiver
Channel B
Transmitter
Channel B
External Status
+
High
IEI
High
IEO
IEI
High
IEO
IEI
High
IEO
IEI
High
IEO
IEI
High
IEO
IEI
High
IEO
1. Priority Interrupt Daisy Chain before Any Interrupt Occurs.
+
High
IEI
High
IEO
IEI
High
IEO
IEI
High
IEO
IEI
2. Channel B Transmitter Interrupts and is Acknowledged.
Under Service
High
IEO
IEI IEO
IEI
Low
IEO
Low
+
High
IEI
High
IEO
IEI
High
IEO
IEI
High
IEO
IEI
Service Suspended
High
IEO
IEI IEO
IEI
Low
3. External/Status Channel A Interrupts Suspending Service of Channel B Transmitter.
IEO
Low
+
High
IEI
High
IEO
IEI
High
IEO
IEI
High
IEO
IEI
Service Resumed
High
IEO
IEI IEO
IEI
Low
4. Channel A External/Status Routine Complete, RETI Issued, Channel B Transmitter Service Resumed.
IEO
Low
+
High
IEI
High
IEO
IEI
High
IEO
IEI
High
IEO
IEI
Service Completed
High
High
IEO
IEI IEO
IEI
High
IEO
5. Channel B Transmitter Service Routine Complete, Second RETI Issued.
Figure 132. Typical Interrupt Service
UM008101-0601
Serial Input/Output