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SureTemp
®
Model 678/SureTemp
®
Model 679 4-1
4. THEORY OF OPERATION
Technical Overview
The heart of the Model 678 and Model 679 is comprised of two integrated circuits
which provide most of the microcontroller and analog circuit functions.
All control and display functions are governed by the microcontroller (U1).
All probe analog data interfacing to the microcontroller is provided by U2.
U2 functions as an A/D converter.
NEC UPD78064 uP
16 bit
Timer
Horn
Driver
Timer
CPU
MODEL 678 PCA
A/D
Converter
Circuit
Voltage
Reference
Circuit
LCD
Driver
Serial
I/O
Serial Port
irDA
Circuitry
Probe
Warmer
Circuit
Liquid
Crystal
Display
Mode Button
Timer Button
PROBE ASSEMBLY
Probe
Warmer
Thermistor
Four
Conductor
Coil Cord
Probe Connector
3 AA Battery
Pack
Probe
Logic
Circuit
Horn
Backlight
Circuit
Battery
Detector
Circuit
8 bit
A/D
Figure 3 - System Block Diagram
Probe resistance measurements are made by ratioing pulse widths which are
generated in U2 by sequentially switching in a calibration resistor and the probe
thermistor. These pulse widths are measured by the microprocessor, which calculates
the probe resistance. The actual probe temperature is then calculated from the probe
resistance.
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