mc68hc05jp6pe Freescale Semiconductor, Inc, mc68hc05jp6pe Datasheet - Page 128

• Manufacturer Part Number: mc68hc05jp6pe
• Description: M68hc05 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Analog Subsystem
8.7 Voltage Measurement Methods
General Release Specification
128
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
reference source, then the unknown voltage will be determined as an
absolute reading. If V
the 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 be indeterminate. In this case, the comparator output flags may also
V
V
N
N
N
REF
X
X
REF
OFF
N
REF
N
FSMAX
FSMIN
= 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
is a stable voltage source, such as a zener or other
V
= C
Analog Subsystem
= C
X
= V
EXTMAX
EXTMIN
REF
REF
DD
is the supply source to the device (V
x (N
x V
x V
–1.5 volts), then the external capacitor will
X
FSMIN
FSMAX
– N
MC68HC05JJ6/MC68HC05JP6
OFF
x f
x f
) / (N
OSCMIN
OSCMAX
SS
REF
and V
/ (P x I
– N
/ (P x I
Freescale Semiconductor
REF
OFF
DD
CHGMAX
)
CHGMIN
or a ratiometric
DD
)
—
REF
)
), then
Rev. 3.2
SS
)
)