XC68HC705JJ7 Motorola, XC68HC705JJ7 Datasheet - Page 132

• Manufacturer Part Number: XC68HC705JJ7
• Description: MICROCONTROLLER
• Manufacturer: Motorola

Analog Subsystem
8.7 Voltage Measurement Methods
Advance Information
132
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:
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:
When V
source, then the unknown voltage will be determined as an absolute
reading. If V
unknown voltage will be determined as a ratio of V
reading.
If the unknown voltage applied to the comparator is greater than its
common-mode range (V
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
V
V
N
N
N
REF
X
X
REF
OFF
N
REF
N
FSMAX
= Known reference voltage
= Unknown voltage between V
= Conversion count for unknown voltage
= Conversion count for known reference voltage (V
= Conversion count for minimum reference voltage (V
FSMIN
is a stable voltage source such as a zener or other reference
REF
V
Analog Subsystem
= C
X
is the supply source to the device (V
= C
= V
EXTMAX
EXTMIN
REF
DD
x (N
x V
–1.5 volts), then the external capacitor will
x V
X
FSMIN
FSMAX
– N
MC68HC705JJ7 • MC68HC705JP7 — REV 4
OFF
x f
x f
) / (N
OSCMIN
SS
OSCMAX
and V
REF
/ (P x I
– N
/ (P x I
REF
DD
OFF
DD
, or a ratiometric
CHGMAX
CHGMIN
), then the
)
REF
MOTOROLA
)
)
SS
)
)