Freescale Semiconductor, Inc.
Analog Subsystem
Voltage Measurement Methods
voltage on the negative input (–). All A/D conversion methods should
have a maximum time check to determine if this case is occurring.
Once the maximum timeout detection has been made, the state of the
comparator outputs can be tested in order to determine the situation.
However, such tests should be carefully designed when using modes 1,
2, or 3 as these modes cause the immediate automatic discharge of the
external ramping capacitor before any software check can be made of
the output state of comparator 2.
NOTE: All A/D conversion methods should include a test for a maximum
elapsed time in order to detect error cases where the inputs may be
outside of the design specification.
8.7.1 Ab solute Volta g e Re a d ing s
The absolute value of a voltage measurement can be calculated in
software by first taking a reference reading from a fixed source and then
comparing subsequent unknown voltages to that reading as a
percentage of the reference voltage multiplied times the known
reference value.
The accuracy of absolute readings will depend on the error sources
taken into account using the features of the analog subsystem and
appropriate software as described in Table 8-6. As can be seen from this
table, most of the errors can be reduced by frequent comparisons to a
known voltage, use of the inverted comparator inputs, and averaging of
multiple samples.
8.7.1.1 Inte rna l Ab so lute Re fe re nc e
If a stable source of V is provided, the reference measurement point
DD
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 AMUX
REF
register. This connects the channel selection bus to the V pin. In order
DD
to stay within the V
range the DHOLD bit should be used to select
MAX
the 1/2 divided input.
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
General Release Specification
Analog Subsystem
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