6.25 Calibration Self-Check Circuitry
The F-2000 performs a two-point calibration self-check every time it is turned on. This is accomplished under a
microcontroller program control by biasing the FET switches Q9 and Q10 in turn, switching precision calibration
resistances into the F-2000 front-end circuitry. Q10 switches in the "Low Cal" resistance value represents
89.5ºF (31.9ºC). Q9 switches in the "Hi Cal" resistance value represent 104.9ºF, (40.5ºC). If actual readings
vary more than 0.2ºF (0.1ºC), the F-2000 will flash the "bAd CAL" message until the problem is corrected.
When the calibration check is completed, the thermistor is switched into the front-end circuitry via Q8 for normal
operation.
If any of the calibration check components (Q9, Q10, R10, R11) fail, an erroneous "bAd CAL" message will be
displayed. A failure in this area can be confirmed if a factory style calibration can be successfully performed
(see Section 7). If the "bAd CAL" message persists, check the calibration circuitry for defective FETS, or
calibration resistors out of tolerance.
If a "bAd Prb" message is displayed when a known good probe is connected to the F-2000, Q8 could be faulty.
6.26 Other Inputs
Switches S3-1 (at U2, PD4), S3-2 (at U3, PD3), S3-3 (at U3, PD5), and S2 (at U3, PD2), are used to select the
pulse timer duration, Fahrenheit or Centigrade mode, calibration mode, and the direct or normal mode
respectively. S3-1, S3-2 and S3-3 are located on the dip switch accessible through the battery compartment. S2
is a slide switch located on the back panel of the F-2000.
The pulse switch (at U3, PD1) is the momentary action membrane switch located on top of the F-2000 display
and has a dual function. When in the direct or predict mode, pressing this button will initiate the pulse timing
function. The F-2000 will display the letter "t" in the most significant digit of the display for the duration of time
set by S3-1 above. When in the calibration mode, pressing this button toggles the low calibration and high
calibration values into the display to visually verify and/or assist in the calibration of the F-2000.
6.27 Seven Segment Display and Audio Alarm
Port B (PB0-PB7) of the MCU is used to control each of four seven-segment displays. Individual seven-
segment displays are multiplexed and controlled by pins PA0-PA3. Each digit is turned on in succession every
four msec.
Pin 25 (PC6) of the MCU is an output line that controls the beeper used to signal that the final temperature is
being displayed. The alarm may optionally be silenced by switching S3-4.
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