MC68HC05JJ6 Datasheet, PDF (129/216 Pages) Freescale Semiconductor, Inc – General Release Specification Microcontrollers

Analog Subsystem
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.
NOTE:
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.
8.7.1 Absolute Voltage Readings
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 Internal Absolute Reference
If a stable source of V_DD is provided, the reference measurement point
can be internally selected. In this case, the reference reading can be
taken by setting the V_REF bit and clearing the MUX1:MUX4 bits in the
AMUX register. This connects the channel selection bus to the V_DD pin.
To stay within the V_MAX range, the DHOLD bit should be used to select
the 1/2 divided input.
MC68HC05JJ6/MC68HC05JP6 — Rev. 3.2
Freescale Semiconductor
Analog Subsystem
General Release Specification
129