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Mode 1: Programmable One-Shot. The output will go low on the
count following the rising edge of the gate input and remain low
until the terminal count.
Mode 2: Rate Generator. The output is initially high, then it
goes low for one cycle. The process then repeats.
Mode 3: Square Wave Generator. high during on half of the count
and low during the other half. for odd count the output stay high
for (N+l)/2 of the count and low for (N-1)/2 of the count.
Mode 4: Software Triggered strobe. The output will go low for
one clock period after the terminal count which is initialized by
the loading of the counter.
Mode 5: Hardware Triggered Strobe. The counter will start
counting after the rising edge of the gate input and the output
will stay low for one clock period at the end of the count. The
counter is retriggerable.
V.6.3
PIC
The Intel 8259 handles up to 8 vectored priority interrupts for
the cpu. It is cascadable for up to 64 vectored interrupts.
The 8259 is designed to minimize the software and real time
overhead in handling multi-level priority interrupts. It has
several modes, permitting optimization for a variety of system
requirement.
To use this device two types of command words should be
defined; the Initialization Command Words CICWs) and the
operation Command Words (OCWs).
ICW1 indicates if that PIC takes place in a multi-pic
configuration or not and defines the level or the edge
triggered input for the interrupts.
ICW2 gives the address of the first vector interrupt.
ICW3 is only used for a slave PIC and indicates to which master
interrupt it is connected.
ICW4 defines whether or not an EOI (End Of Interrupt) is
generated and if the Fully Nested Mode is used (Interrupt
requests are ordered from 0 trough 7).
Concerning the Operation Command Words, only OCW1 has to be
clearly defined because it sets the interrupt mask. The others
can be forgotten in most of applications.
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