Peripherals To U1; Microcomputer Control - Fluke 27 Service Manual

27
Service Manual
preset, and bi-directional bus control for storing control outputs from the microcomputer
and for transferring data to the microcomputer.
Basic timing for the A/D converter is defined as a series of 10 integrate/read cycles
(samples), followed by a 100 ms autozero phase. However, the diode test and continuity
function, the 32 MJ range, the battery test, the power-up self test, overload recovery,
autoranging, and the Touch-Hold
The state machine, in combination with the ROM and preset read counter, plus an
autozero flag under computer control, establishes the timing variances necessary for the
various functions.
2-8.

Peripherals to U1

Circuitry peripheral to U1 provides regulated battery voltage to power U1, a regulated
reference voltage for the A/D converter, a system clock, signal conditioning, and
amplifier stabilization. The battery voltage regulator consists of AR1, Q12, and
associated components; the regulator circuit supplies consistent operating power to U1
and, through a second regulator circuit in U1, to the microcomputer (U2). Voltage
regulator VR1 (and associated components) supplies a regulated 1.000 V reference
voltage for the A/D converter. Potentiometer R19 provides for calibration adjustment of
the reference voltage.
Additional circuits are necessary for the active filter, integrator, and buffer in U1. the
active filter response is determined by R13, R16, C18, and C19. Integrator and buffer
signal characteristics are determined by C20, C21, and two resistors in Z1. Several
components external to U1 provide for ac-to-dc conversion; they are C41, C42, C43,
R35, R7, C16, R30, R31, and R32.
The system clock, with a frequency of 32.768 kHz, controls all timing synchronization
for the instrument. Y1 is a quartz crystal which determines the frequency of the clock
oscillator circuit.
2-9.

Microcomputer Control

A CMOS, 4-bit microcomputer (U2) controls the various instrument functions and drives
the LCD display. The microcomputer reads and processes data samples from the A/D
converter, sends a code to U1 which represents the operator-selected function, performs
the Touch-Hold algorithm, selects the correct A/D mode for the function in used,
controls range, sets the autozero flag, and disables the analog filter during autoranging.
In reading and processing A/D samples, the computer accepts raw sample data, applies
necessary corrections as described in the preceding discussion of U1, and accumulates
10 samples which then become the full resolution conversion for digital display. Each
minor conversion is also processed for the bar-graph display. For the diode test and
continuity function, the microcomputer evaluates the data and determines whether or not
the beeper should be switched on.
Touch-Hold operation requires the microcomputer to perform a different algorithm. The
microcomputer does not allow a full-resolution conversion to be completed unless the
input signal is stable. When a stable reading occurs, the conversion is completed and the
microcomputer generates the corresponding display and freezes it. the microcomputer
then waits for a change in the signal to exceed a certain threshold, and then begins
watching for a stable reading again. There are two exceptions to this simple algorithm:
First, open test lead indication does not allow a full-resolution conversion to be
completed either; the microcomputer continues to wait for a stable signal which is
outside the open test lead region. (Open test leads in voltage or current function result in
low readings; open test leads in resistance or diode test functions result in off-scale
2-6
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mode all required variations from the basic timing.