XC68HC705JJ7 Motorola, XC68HC705JJ7 Datasheet - Page 133

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

8.7.1 Absolute Voltage Readings
8.7.1.1 Internal Absolute Reference
MC68HC705JJ7 • MC68HC705JP7 — REV 4
MOTOROLA
NOTE:
be indeterminate. In this case the comparator output flags may also be
set even if the actual voltage on the positive input (+) is less than the
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 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.
All A/D conversion methods should include a test for a maximum
elapsed time to detect error cases where the inputs may be outside of
the design specification.
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, 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.
If a stable source of V
can be internally selected. In this case, the reference reading can be
taken by setting the V
register. This connects the channel selection bus to the V
within the V
divided input.
MAX
Analog Subsystem
range, the DHOLD bit should be used to select the 1/2
REF
DD
is provided, the reference measurement point
bit and clearing the MUX1:4 bits in the AMUX
Table
8-6. As can be seen from this
Voltage Measurement Methods
Advance Information
Analog Subsystem
DD
pin. To stay
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