68HC705SP7 datasheet(131/230 Pages) FREESCALE

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Analog Subsystem

Voltage Measurement Methods

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

General Release Specification

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 VDD, 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 MUX1:4 bits in the

AMUX register which connects the channel selection bus to the VDD pin.

In order to stay within the VMAX 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–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 VDD –1.5 volts, then the DHOLD bit should be

used to select the 1/2 divided input.

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