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1.1
Microcontroller
The freezing rain sensor uses an Intel 87C51-type microcontroller
to control the freezing rain sensor functions. This 8-bit
microcontroller requires at least: 4 Kbytes of on-board ROM, 128
bytes of RAM, and 32 input/output ports. The freezing rain
sensor uses about 75% of these resources. Upgraded
microcontrollers that provide more resources are available. The
microcontroller runs at 7.372 MHz.
1.2
Watchdog/Reset Circuit
The watchdog timer/reset circuit monitors the microcontroller
and provides a reset pulse if not periodically toggled. The
watchdog also provides reset pulses on initial power-up and holds
the microcontroller in the reset state if the internal power supply
falls below an acceptable voltage. The watchdog indicates
impending power loss so the ice detector can shut down in a
known manner.
1.3
Serial EEPROM
The Serial EEPROM stores unit status (icing state, failure state,
heater state, correlation count) which is recovered after power
interruptions of 200 ms or less. This allows the unit to meet the
power interruption requirements of RTCA DC-160C, Section 16,
Category Z. Additionally, the Serial EEPROM stores
environmental and failure information such as unit elapsed-time,
number of icing encounters, number of failures, and detailed
information on types and quantities of each annunciated failure.
This information is used by Rosemount Aerospace to confirm
and repair failures reported by the end user and also to collect
MTBF data. Each time the Serial EEPROM is written, a
checksum is computed and written. Each time the Serial
EEPROM is read, a checksum is computed and compared to the
stored value.
1.4
Probe Oscillator
The probe oscillator is the electronic control portion of the
magnetostrictive oscillator (MSO) used to sense and detect ice.
This circuit provides the drive and feedback of the ice sensing
probe. The circuit drives the probe at a nominal 40kHz, and
converts the feedback into a CMOS compatible square wave that
is measured by the microcontroller. As ice accretes on the probe,
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