Freescale Semiconductor, Inc.
Ana log Sub syste m
8.7 Volta g e Me a sure m e nt Me thod s
The methods for obtaining a voltage measurement can use software
techniques to express these voltages as absolute or ratiometric
readings.
In most applications the external capacitor, the clock source, the
reference voltage and the charging current may vary between devices
and with changes in supply voltage or ambient temperature. All of these
variations must be considered when determining the desired resolution
of the measurement. The maximum and minimum extremes for the full
scale count will be:
N
= C
x V
x f
/ (P x I
)
CHGMAX
FSMIN
EXTMIN
FSMIN
OSCMIN
N
= C
x V
x f
/ (P x I
)
CHGMIN
FSMAX
EXTMAX
FSMAX
OSCMAX
The minimum count should be the desired resolution; and the counting
mechanism must be capable of counting to the maximum. The final
scaling of the count will be by a math routine which calculates:
V = V
x (N - N
) / (N
- N
)
X
REF
X
OFF
REF
OFF
Where:
V
V
= Known reference voltage
= Unknown voltage between V and V
REF
X
SS
REF
N
N
N
= Conversion count for unknown voltage
= Conversion count for known reference voltage (V
= Conversion count for minimum reference voltage (V )
X
)
REF
REF
OFF
SS
When V
is a stable voltage source such as a zener or other reference
REF
source, then the unknown voltage will be determined as an absolute
reading. If V is the supply source to the device (V ), then the
REF
DD
unknown voltage will be determined as a ratio of V , or a ratiometric
DD
reading.
If the unknown voltage applied to the comparator is greater than its
common-mode range (V –1.5 volts), then the external capacitor will
DD
try to charge to the same level. This will cause both comparator inputs
to be above the common-mode range and the output of the comparator
will be indeterminate. In this case the comparator output flags may also
be set even if the actual voltage on the positive input (+) is less than the
General Release Specification
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
Analog Subsystem
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