68HC705SP7 datasheet(129/230 Pages) FREESCALE

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Analog Subsystem

Voltage Measurement Methods

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

General Release Specification

Analog Subsystem

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 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 VDD is provided, 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

to stay within the VMAX range the DHOLD bit should be used to select

the 1/2 divided input.

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