Star Tech Journal

STAR*TECH JOURNAL/DECEMBEtMNUFACTs
O
7
ATARI POWER-ON RESET AND WATCHDOG CIRCUITS
By Frank Becker, Atari, Inc., Somerset, NJ
The purpose of the Power-On Reset Circuit is
to hold the reset line to the MPU low until the
power supply five volts stabilizes. This will
insure that only valid information is present on
the address and data lines when the MPU is
initialized.
The OP-AMP RS is configured as a differ-
ential input or difference amplifier; this means
it amplifies the difference between both input
pins. In analyzing this particular circuit, we see
a voltage divider network consisting of resistors
R69 (22K) and R72 (lOK) tied to 10.3 volts.
This produces approximately 7 volts on Pin 3,
the non-inverting input, when the 10.3 volts
reaches its maximum value. The inverting
input, Pin 2, has 5 volts applied to it.
Looking at Figure 1, we have 7 volts on the
positive input and 5 volts on the negative input.
We then perform more simple math to find out
if the differenc between the inputs is positive or
negative. 7 - 5 = 2. Because the difference
between the inputs is positive, the OP-AMP's
output will try to go as high as it can; however,
it will be limited to the OP-AMP's positive
power supply which is 10.3 volts. If the difference
had been negative, the output would have gone
as far negative as possible, in this case only
down to ground.
To summarize the operation of the OP-
AMP in this configuration, it can be stated that:
If the difference between the inputs is
positive, the output will be + Vee.
If the difference between the inputs is
negative, the output will be - Vee.
Refer to Figure 2 on page 8. You might now
expect to find the output of RS Pin 1 at 10.3
volts, and you would if it wasn't for the 220-
ohm resistor R73; R73 keeps the output at
about 5 volts. Diode CR3 protects the input to
F7 by clamping the input at 5 volts, should the
OP-AMP's output rise above 5 volts.
The output, Pin 1, will then be 5 volts
except during power-up. During power-up the
output will stay low for a very short time to
allow the five volts to stabilize. This occurs
because the resistor values of the voltage
divider are chosen to insure that Pin 3 never
goes higher than Pin 2 until the 5 volts stabilizes.
(Continued on next page.)
10k
Figure 1
68k
220A.
R72
22k
m
m
10.3
7v
R69
Vout
5.0v, >-----2---1
TO HAVE
T.
MODEL 125 • ARCADE JOYSTICK
The only sensible replacement
• manufac;tured from aluminum stru
• all mechanical fasteners of aircraft ty
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• 2,4, or 8 positions
This unit ~ designed to give you and your
joy possible.
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7330 ADAMS STREET
PARAMOUNT, CA 90723
(213) 633-4494
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