Power Supply; Microcontroller; Reset/Self Tests - Welch Allyn SureTemp 986 Service Manual

A.1.4 Power Supply
A.1.5 Microcontroller
A.1.6 Reset/Self Tests
Model 986 Thermometer
The shape of the rising temperature curve is monitored and the
best fit to a curve is found. When the curve fit is stable, the final
predicted temperature is displayed.
If the prediction criteria explained above are not met within 15
seconds of starting the prediction process, the instrument will
automatically switch to Monitor mode. If the ambient temperature is
above 33.9°C (93.0°F) the unit will automatically switch to Monitor
mode.
Note: Rectal probes are not pre-warmed.
Power is drawn from the two AAA alkaline cells directly to the
circuit electronics. The voltage from the batteries is unregulated
but filtered by capacitor C25. The power supply voltage will range
from about 3.2 volts with new batteries to 2.6 volts at shutdown.
The thermometer has two battery voltage indicators:
•
The first is a warning that batteries are getting low (2.8 volts)
and is indicated by the battery icon flashing in the display.
Accuracy is not affected during low battery warning indication.
•
When the batteries fall to approximately 2.6 volts, the dead
battery error condition is defined to exist. Operation is halted
and the E2.1 error message is displayed. At this point, the
batteries must be replaced and the thermometer electronics
reset. See Reset Self/Tests, Operational Checks, and Battery
Replacement.
An NEC UPD789407 single chip microcontroller in a QFP package
(U1) is used for signal digitizing, data processing, program
memory addressing and storage, and I/O interfacing. The
microcontroller also includes an LCD controller/driver that allows
internal conversion of CMOS logic levels to a data format capable
of driving the Model 986 LCD. In this application, the
microcontroller is running at 2.4576 MHz.
Upon battery installation, (assuming that the electronics have been
discharged sufficiently by pressing the Mode button with batteries
removed) the microprocessor receives a power up reset signal
from the components associated with the reset line U1-67. When
power is applied continuously, C3 is charged slowly through R7
providing an active low reset to the microprocessor.
Appendix A. Theory of Operation
31