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36
_______________________________________________________________ User Manual for the
NOTE:
FF
SET/RESET' LOW
VCC
RUN
t
Q I - - - - - - '
VCC
VCC
r---------
I
4007
I
X ) - - - - - - - ,
I
I
I
I
I
22M
SCI
------1
I
1
I
I
1
CLOCK
I
1
1
20PF
1 2 0 P F
1
1_
-= _____
~
________
---1
COS MAC
92CM-26473
Fig. 43 - Two-switch COSMAC control.
Turning on the RUN switch starts the clock and puts a high on the CLEAR line. Fig. 44 shows the
sequence of events that initiates program execution when the RUN switch is turned on. The clock
cau"ses a TPA signal each machine cycle. The low on the DMA-OUT line is detected by COSMAC_ It
responds by performing a DMA cycle (S2), which is described in the section on I/O interface. A low on the
state code line (SCI) indicates that COSMAC is executing the DMA cycle (or interrupt cycle, which would
not normally occur at this time) and causes the flip-flop holding the DMA-OUT line low to be
~In
this
case, the DMA cycle does nothing more than take COSMAC out of the IDLE state. Since the LOAD line is
high, the cycle immediately following the DMA cycle will be a normal instruction fetch operation (SO).
RUN SWITCH
OFF
w..
ON
CLEAR (LOWI _____ ---'
CLOCK
TPA
DMA-OUT
L..J
I
,I
I
L
~
I----IDLE CYCLE
(SII----1.~I
..
o__-
DMA-OUT CYCLE ( S 2 1 - - 1
92CM-26471
Fig. 44 - START timing.
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