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6 Safety Features and
Fault Detection
Hardware Safety Features
Key hardware safety features include a watch-
dog timer circuit, motor driver and motor
watchdog circuits, cassette ‘type’ sensor circuit,
latch/lock sensor circuit, and a voltage detector
circuit. Each safety circuit performs a unique
function to insure the overall safety of the
device. (See Figure 8.)
Watchdog Timer Circuit
The microprocessor must send an appropriate
signal to the watchdog circuit at least once per
second. If the microprocessor does not, the
watchdog circuit will time out and shut down
the pump controller.
Watchdog timer circuitry is provided to monitor
the status of the microprocessor and disable
the motor and enable the audible alarm if the
microprocessor fails to function properly. The
microprocessor must strobe the watchdog
circuit at least once every second in order to
prevent the watchdog from performing its reset
function. The reset output from the watchdog
circuit is a pulse output. This acts to “jump
start” the microprocessor. This unique feature
allows the microprocessor to test the watchdog
circuit on every power-up. By setting a flag in
memory and not strobing the watchdog, the
microprocessor can force a watchdog time-out.
After being reset, the microprocessor checks
the status flag to see if this was a time-out test.
If so, the microprocessor continues normal
power-up activities. If the reset occurred when
the microprocessor was not expecting it, the
microprocessor traps the event, sounds the
audible alarm and displays an error message
on the LCD.
Figure 8. CADD-Prizm
®
pump hardware block diagram.
PROGRAM
MEMORY
MB
DATA
MEMORY
MB
LCD DISPLAY
VOLTAGE
REFERENCE
MOTOR
DRIVER
WATCHDOG
REAL-TIME
CLOCK
CPU/IO/
GATE ARRAY
KEYBOARD
MOTOR
WATCHDOG
VOLTAGE
DETECTOR
SENSORS
AUDIBLE
ALARM
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