Freescale Semiconductor, Inc.
Analog Subsystem
Voltage Measurement Methods
8.7.2 Ra tiom e tric Volta g e Re a d ing s
The ratiometric value of a voltage measurement can be calculated in
software by first taking a reference reading from a reference source and
then comparing subsequent unknown voltages to that reading as a
percentage of the reference value. The accuracy of ratiometric readings
will depend on the variety of sources, but will generally be better than for
absolute readings. Many of these error sources can be taken into
account using the features of the analog subsystem and appropriate
software as described in Table 8-7. As with absolute measurements,
most of the errors can be reduced by frequent comparisons to the
reference voltage, use of the inverted comparator inputs, and averaging
of multiple samples.
8.7.2.1 Inte rna l Ra tio m e tric Re fe re nc e
If readings are to be ratiometric to V , the reference measurement
DD
point can be internally selected. In this case the reference reading can
be taken by setting the V
bit and clearing the MUX1:4 bits in the
REF
AMUX register which connects the channel selection bus to the V pin.
DD
In order to stay within the V
range the DHOLD bit should be used to
MAX
select the 1/2 divided input.
8.7.2.2 Exte rna l Ra tio m e tric Re fe re nc e
If readings are to be ratiometric to some external source, the reference
measurement point can be connected to any one of the channel selected
pins from PB1–PB4. In this case, the reference reading can be taken by
setting the MUX bit in the AMUX which connects channel selection bus
to the pin connected to the external reference source. If the external
reference is greater than V –1.5 volts, then the DHOLD bit should be
DD
used to select the 1/2 divided input.
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
General Release Specification
Analog Subsystem
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