Theory of Operation
Functional Description
4
4-5
After the power-on self-test routine is successfully completed, the
microcomputer checks to see if the operator has selected the ratio self-test or
the switch decoding self-test (refer to Chapter 5 for operating instructions). If
neither of the self-tests has been selected, the microcomputer begins the
normal operating routine. The operating routine consists of four steps:
1. The microcomputer reads the function and range selections and checks
the four push buttons to determine the mode the operator has selected.
The microcomputer then selects either the a/d converter (for
measurement of voltage, current, resistance, conductance, continuity, or
the diode test) or the frequency counter.
2. The microcomputer initiates either the a/d measurement cycle
(approximately 400 ms) or the frequency measurement cycle
(approximately 1.0s). The measurement cycles are described later in this
chapter.
3. The microcomputer processes the data obtained in the measurement
cycle. This includes calculations for the dB, relative (REL) offset, and
MΩ or frequency autoranging.
4. The microcomputer displays the results. The results remain on the
display until it is updated.
After the results are displayed, the routine begins again at the first step.
4-4. Measurement Acquisition Chip (MAC)
A block diagram of the MAC is shown in Figure 4-1. The digital control
logic includes a buffer and decoder, read and write logic, status and control
registers, and logic control for the continuity function. The power supply
control uses the calibrated 1V a/d reference voltage obtained from a bandgap
reference diode to regulate the 5.2V main power supply for the instrument.
When the continuity function is selected and continuity is detected, the MAC
generates the tone by supplying a square wave to the external piezoelectic
transducer.
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