Legacy® PLUS - Smiths Medical
Legacy® PLUS - Smiths Medical
Legacy® PLUS - Smiths Medical
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Reference<br />
Section 5: Reference<br />
Safety Features and Fault Detection<br />
Hardware Safety Features<br />
Key hardware safety features include a watchdog timer circuit, motor<br />
driver and motor watchdog circuits, and a voltage detector circuit. Each<br />
safety circuit performs a unique function to insure the overall safety of<br />
the device.<br />
Watchdog Timer Circuit<br />
The microprocessor must send an appropriate signal to the watchdog circuit<br />
at least once per second. If the microprocessor does not, the watchdog<br />
circuit will time out and shut down the pump controller.<br />
Watchdog timer circuitry is provided to monitor the status of the microprocessor<br />
and disable the motor and enable the audible alarm if the<br />
microprocessor fails to function properly. The microprocessor must<br />
strobe the watchdog circuit at least once every second in order to prevent<br />
the watchdog from performing its reset function. The reset output<br />
from the watchdog circuit is a pulse output. This acts to “jump start” the<br />
microprocessor. This unique feature allows the microprocessor to test the<br />
watchdog circuit on every power-up.<br />
By setting a flag in the memory and not strobing the watchdog, the<br />
microprocessor can force a watchdog time-out. After being reset, the<br />
microprocessor checks the status flag to see if this was a time-out test. If<br />
so, the microprocessor continues normal power-up activities. If the reset<br />
occurred when the microprocessor was not expecting it, the microprocessor<br />
traps the event, sounds the audible alarm and displays an error<br />
message on the LCD.<br />
Motor Driver/Motor Watchdog Circuit<br />
Motor drive circuitry is composed of a series of power FET transistors,<br />
passive components, and 2 voltage comparators. Built into the motor<br />
drive circuitry is an RC timer which times how long the motor runs<br />
each time it is turned on. If the motor runs for more than an average<br />
of 3 seconds, the circuit will time out and disable the motor. A unique<br />
feature of this circuit is that control lines to and from the microprocessor<br />
circuit allow the microprocessor to perform a complete functional test of