of Microprocessors

260 MUSICAL ApPLICATIONS OF MICROPROCESSORS
CONTROLLED BY
OUTPUT PORT BIT
C
VOLTAGE
TO
MEASURE
Fig. 7-27. Single-slope integration method
microcomputer opens the switch and monitors the output of the comparator
as before. The integrator output now starts going positive at a constant rate
determined by the reference voltage magnitude. When it finally becomes
more positive than the unknown voltage, the comparator switches low and
the accumulated count is the converted value. As before, the resolution
attainable is dependent on the speed of the microcomputer and the time
available for conversion.
Dual-Slope Method
One problem with both of the preceding circuits is that accuracy and
stability depend on the quality of a capacitor. The circuit in Fig. 7-28
overcomes this difficulty by comparing the input voltage with a reference
voltage, using the capacitor only as a comparison medium. A measurement
cycle consists of two phases. Normally S2 is on, forcing the integrator output
to zero. During phase 1, 51 selects the unknown voltage for the integrator
input, and S2 is opened to allow the integrator to charge negative at a rate
dependent on the unknown voltage. The amount of time spent in phase 1 is
constant and carefully controlled by a counting loop similar to that used for
measurement. At the beginning of phase 2, Sl is flipped so that the negative
S2
VOLTAGE TO0--0 SI
MEASURE
I
TO INPUT
PORT BIT
Fig. 7-28. Dual-slope integration method