MC68HC705JP7 Datasheet, PDF (132/242 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Analog Subsystem
8.7 Voltage Measurement Methods
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_FSMIN = C_EXTMIN x V_FSMIN x f_OSCMIN / (P x I_CHGMAX)
N_FSMAX = C_EXTMAX x V_FSMAX x f_OSCMAX / (P x I_CHGMIN)
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:
Where:
V_X = V_REF x (N_X – N_OFF) / (N_REF – N_OFF)
V_REF = Known reference voltage
V_X = Unknown voltage between V_SS and V_REF
N_X = Conversion count for unknown voltage
N_REF = Conversion count for known reference voltage (V_REF)
N_OFF = Conversion count for minimum reference voltage (V_SS)
When V_REF is a stable voltage source such as a zener or other reference
source, then the unknown voltage will be determined as an absolute
reading. If V_REF is the supply source to the device (V_DD), then the
unknown voltage will be determined as a ratio of V_DD, or a ratiometric
reading.
If the unknown voltage applied to the comparator is greater than its
common-mode range (V_DD –1.5 volts), then the external capacitor will
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
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MC68HC705JJ7 • MC68HC705JP7 — REV 4
Analog Subsystem
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