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350 Cable Survey System
DPN 402197 © TSS (International) Ltd Page 10 of 18
The output of the receiver is fed to a comparator which has two threshold settings set
by the microcontroller. The initial threshold is set approximately 4dB higher for this
period thus allowing echo signals to be received even when direct signals are still
being received from the effect of transducer ringing.
The detected receiver signal is fed to a capture timer on the microcontroller which
stops the timer on the negative edge of the received pulse.
The same timer channel is also used to read the temperature/depth transducer; this is
carried out by a multiplexer connected to both circuits and controlled by the microcon-
troller.
D.3.2.4 Sensor Circuitry
The sensor circuitry is a complete front end for the measurement of passive sensors
such as temperature and pressure using an advanced chopping technique to remove
low frequency interference. The analogue sensor signal is converted into a digital for-
mat and read by the microcontroller. The sensor interface converts to 12 bit accuracy.
A three phase technique is used to measure system offset, reference and finally the
sensor signal. Good long term stability is assured by the auto calibration process car-
ried out by the microcontroller to determine changes in offset and reference for each
measurement period.
The sensor circuitry is located close to the sensor to reduce resistance between sen-
sor and electronics; however, even the small resistance changes due to temperature
effect etc. are corrected by software using the offset and reference measurements.
The chopping technique filters out any spurious noise generated by internal or exter-
nal circuits.
The accuracy of the sensor is determined by the accuracy of the temperature or
depth sensor itself and the reference resistor R1. Corrections can also be made in the
microcontroller to achieve greater accuracy. The reference resistor is currently a
100R resistor 0.01% < 0.6ppm/ºC accuracy which equates to an initial accuracy of
±0.01R or 0.026ºC plus temperature drift which is negligible.
D.3.2.5 Digital Circuitry
The digital circuitry comprises the microcontroller and serial communication plus
associated circuitry. The microcontroller operates at a speed of 8MHz and is used to
process all transmission and receive functions in conjunction with the associated
hardware.
The firmware for the microcontroller, (68HC908KX8), can be programmed using the
mini “MONO8” connector at J4. This connector allows the micro to be programmed in-
situ using its flash memory to retain the data when power is removed.
Standard tools from Motorola are available that allow the manufacturer to program via
this connection without the need to connect to the power supply.
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