MC68HC05JJ6 Datasheet, PDF (131/216 Pages) Freescale Semiconductor, Inc – General Release Specification Microcontrollers

Analog Subsystem
8.7.2 Ratiometric Voltage Readings
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 Internal Ratiometric Reference
If readings are to be ratiometric to V_DD, the reference measurement
point can be internally selected. In this case, the reference reading can
be taken by setting the VREF bit and clearing the MUX1:MUX4 bits in
the AMUX register, which 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.
8.7.2.2 External Ratiometric Reference
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 through 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_DD –1.5 volts, then 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
131