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THEORY OP OPERATION
Program control of the PIC-8 board is done entirely through
one output port location.
is jumper-selected in-socket positions E4 and E5, and
forms the input to the 8 input NAND gate (741s30) .
output of this address select is ANDed with the Processor
Write Strobe and Phase II clock and provides an output
strobe which is used to latch the lower 4 bits of output
data into the 8214 priority interrupt chip, and the upper
4 bits into the 7475 4 bit latch.
When the 8214 is ENABLED and one of the priority request
lines is low the 8214 sets the output of a 2 GATE Flip-Flop
low to request an interrupt from the 8080 processor.
the processor acknowledges the interrupt the Flip-Flop
reset and 3 buffer drivers of the 8T98 are enabled to put
interrupt request address on bits 3, 4, and 5 of the DATA
IN bus.
The remaining bits of the DATA are not driven, and
remain high via pullup resistors on the MPU Board.
bYte thus formed on the DATA IN bus is a restart instruction,
with bits 3, 4, and 5 disecting the processor to one of
eight restart locators.
Further details on the theory of operation of the 8214
chip can be found in the Intel Data Book.
The PIC-8 board also includes a software controlled
interval clock.
running at two megahertz and divides it by 200 using a
divide-by-two (7474) followed by two divide-by-10 sections
(7490) to provide the ,1 millisecond intervals.
Four consecutive divide-by-10 7490's are then used to
produce the other interval rates up to the longest rate of
one second.
Jumper selection is made from among these
rates and ANDed with the output port bits 4, 5 and 6 and
the output from the AND gate is used to drive the clock
on the other half of the 7474 D type flip-flop.
of the flip-flop is connected so that on successive clocks
it will shift states and thus alternately request and
remove the request for an interrupt.
When the processor system is running, and replying to the
interrupts, shortly after the request is issued, the
interrupt acknowledge line will become active in the low
state and set this flip-flop to remove the interrupt
.PIC-8
The address of this output port
The clock circuit takes the Phase II clock
t ' i e - ö
Theory of Operation
Revision 3
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