Interrupt Operation
VL-486-4 Reference Manual
CIO Chip – 99
Interrupt Operation
O
VERVIEW
Interrupts are generated whenever CPU intervetion is required by a peripheral device. Two
examples of interrupt request sources in the CIO are: a pattern match occurring in a bit port or a
counter/timer reaching its terminal count.
P
RIORITY
H
ANDLING AND THE
CIO
The CIO is designed to provide interrupt priority resolution in situations where there may be
competing interrupt requestors.
The CIO has five potential sources of interrupts: the three counter/timers and Ports A and B. The
priorities of these sources are fixed in the following order (highest to lowest): Counter/Timer 3,
Port A, Counter/Timer 2, Port B, and Counter/Timer 1.
Interrupts generated by the CIO are routed to the VL-486-4 interrupt controller on IRQ15 via
jumper block V13[1-2].
T
HE
F
OUR
I
NTERRUPT
L
OGIC
F
UNCTIONS
The CIO has the logic necessary to: generate interrupts, resolve priority when there is more than
one interrupt requestor, inhibit preemptive interrupts by the lower-priority requestors, and clearly
identify the exact source of interrupt.
G
ENERATING THE
I
NTERRUPT
R
EQUEST
Each source interrupt in the CIO contains three bits for the control and status of the interrupt
logic: an Interrupt Pending (IP) bit, an Interrupt Under Service (IUS) bit, and an Interrupt Enable
(IE) bit. IP is automatically set when an event requiring CPU intervention occurs. The setting of
IP results in an Interrupt Request on IRQ15 via jumper block V13[1-2]. IP can also be set by a
command. This is useful when debugging interrupt handler software.
The IE bit provides a means of masking off individual sources of interrupts within the CIO.
When IE is set to 1, an interrupt request is generated normally. When IE is reset to 0, the IP is
masked off. The IP bit is still set when an event occurs that would normally require service;
however a hardware interrupt request is not generated.
The IUS status bit is set by the CPU as a result of the Interrupt Acknowledge cycle if, at the time
of the Interrupt Acknowledge cycle, the corresponding IP is the highest-priority unmasked IP.
When IUS is 1, it indicates that the corresponding IP has been recognized by the CPU and is
being serviced. As long as IUS is set the corresponding IP is masked off and a hardware interrupt
request is generated.
The Master Interrupt Enable (MIE) bit allows all sources of interrupts within the CIO to be
disabled without having to individually clear each IE to 0. If MIE is reset to 0, all IPs are masked
off and no interrupt can be requested or acknowledged.