68HC705SP7 datasheet(128/230 Pages) FREESCALE

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Analog Subsystem

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Analog Subsystem

8.7 Voltage Measurement Methods

The methods for obtaining a voltage measurement can use software

techniques to express these voltages as absolute or ratiometric

readings.

In most applications the external capacitor, the clock source, the

reference voltage and the charging current may vary between devices

and with changes in supply voltage or ambient temperature. All of these

variations must be considered when determining the desired resolution

of the measurement. The maximum and minimum extremes for the full

scale count will be:

NFSMIN = CEXTMIN x VFSMIN x fOSCMIN / (P x ICHGMAX)

NFSMAX = CEXTMAX x VFSMAX x fOSCMAX / (P x ICHGMIN)

The minimum count should be the desired resolution; and the counting

mechanism must be capable of counting to the maximum. The final

scaling of the count will be by a math routine which calculates:

VX = VREF x (NX - NOFF) / (NREF - NOFF)

Where:

VREF = Known reference voltage

= Unknown voltage between VSS and VREF

= Conversion count for unknown voltage

NREF = Conversion count for known reference voltage (VREF)

NOFF = Conversion count for minimum reference voltage (VSS)

When VREF is a stable voltage source such as a zener or other reference

source, then the unknown voltage will be determined as an absolute

reading. If VREF is the supply source to the device (VDD), then the

unknown voltage will be determined as a ratio of VDD, or a ratiometric

reading.

If the unknown voltage applied to the comparator is greater than its

common-mode range (VDD –1.5 volts), then the external capacitor will

try to charge to the same level. This will cause both comparator inputs

to be above the common-mode range and the output of the comparator

will 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

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