Z86C61 Datasheet, PDF (25/46 Pages) Zilog, Inc. – CMOS Z8 MICROCONTROLLER

Interrupts. The Z86C61/62/96 has six different interrupts
from eight different sources. The interrupts are maskable
and prioritized. The eight sources are divided as follows:
four sources are claimed by Port 3 lines P33-P30, one in
Serial Out, one is Serial In, and two in the counter/timers
(Figure 23). The Interrupt Mask Register globally or indi-
vidually enables or disables the six interrupt requests.
When more than one interrupt is pending, priorities are
resolved by a programmable priority encoder that is con-
trolled by the Interrupt Priority register. All Z86C61/62/96
interrupts are vectored through locations in the program
memory. When an interrupt machine cycle is activated, an
interrupt request is granted. Thus, this disables all of the
subsequent interrupts, saves the Program Counter and
Status Flags, and then branches to the program memory
vector location reserved for that interrupt. This memory
location and the next byte contain the 16-bit address of
the interrupt service routine for that particular interrupt
request.
Z86C61/62/96
Z8^® MICROCONTROLLER
To accommodate polled interrupt systems, interrupt in-
puts are masked and the Interrupt Request register is
polled to determine which of the interrupt requests
need service. Software initialed interrupts are supported
by setting the appropriate bit in the Interrupt Request
Register (IRQ).
Internal interrupt requests are sampled on the falling edge
of the last cycle of every instruction. The interrupt request
must be valid 5TpC before the falling edge of the last clock
cycle of the currently executing instruction.
For the ROMless mode, when the device samples a valid
interrupt request, the next 48 (external) clock cycles are
used to prioritize the interrupt, and push the two PC bytes
and the FLAG register onto the stack. The following nine
cycles are used to fetch the interrupt vector from external
memory. The first byte of the interrupt service routine is
fetched beginning on the 58th TpC cycle following the
internal sample point, which corresponds to the 63rd TpC
cycle following the external interrupt sample point.
IRQ0 - IRQ5
IRQ
Interrupt
Request
IMR
6
Global
Interrupt
Enable
IPR
PRIORITY
LOGIC
Vector Select
Figure 23. Interrupt Block Diagram
25