Welch Allyn SureTemp 670 Spezifikationen PDF herunterladen (Seite 26)

Technical Manual

4-4 Welch-Allyn, Inc.

Reset/Self Tests

Upon battery installation, (assuming that the electronics have been discharged

sufficiently by pressing a user button with batteries removed) the microprocessor

receives a power up reset signal from the components associated with the reset line at

U1-12. When power is applied continuously, C3 is charged slowly through R7,

providing an active low reset to the microprocessor.

When the reset signal is complete, the microprocessor launches a series of self

checks which include RAM test, ROM test, instruction set test, self calibration tests

(electronics accuracy test, hi cal, low cal), probe warmer circuitry tests, probe test,

battery voltage test and ambient temperature test. Any failures here will cause a

specific error code to be displayed to assist debugging.

Microprocessor Clock

The clock for the microprocessor is generated by X1 and capacitors C26 and C27,

which form a 4.9152 MHz. oscillator circuit. The microcontroller is running at

approximately 2.5 MHz, which is achieved by internally dividing the frequency of

oscillation by two.

Temperature Measurement and Display

The thermometer probes incorporate negative temperature coefficient thermistors.

When the temperature of the probe is increased, its electrical resistance decreases.

Model 678 and 679 use “20K” thermistors, so they are at approximately 20 Kohms at

room temperature. At 37

C (98.6

F) they are near 12 Kohms. The change in

resistance is nonlinear with temperature, and an equation describing this curve is

programmed into the thermometer.

Temperature Measurement A/D Converter

The primary function of the temperature measurement A/D converter circuit is to

convert a measured resistance into a 16-bit word, which is then used by the

microprocessor to calibrate itself and calculate a temperature.

Theory of Operation

The A/D circuit is made up of 4 major parts:

•

The resistance to be measured (either the probe thermistor, the PTBCAL resistor,

or the HICAL resistor).

•

The timing capacitor (C21, 0.33uF) and associated driver circuitry

(Q1-Q4, Q9, R15, and R30).

•

A low bias dual voltage comparator (U2, TLC555).

•

A 16 bit timer internal to the microprocessor.

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