Campbell Scientific 0871LH1 Freezing Rain Sensor - Manual (Page 27)

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1 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, the frequency decreases,
and it is this frequency change that the microcontroller annunciates in
the form of Ice Signal #1.
1.5 Heater Control
The heater control turns the probe heater on and off as commanded by
the microcontroller and monitors the actual heater state (ON or OFF) for
verification by the microcontroller. Two outputs are required from the
microcontroller to turn on the heater. This minimizes the possibility of an
unintended heater ON condition. The heater control also monitors the
state of the heater and provides feedback to the microcontroller so that it
can be determined whether the heater is on or off.