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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