PIC18F23K22-ESLASHML datasheet(325/496 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

Preliminary

DS41412C-page 325

PIC18(L)F2X/4XK22

19.4

Measuring Capacitance with the

CTMU

There are two separate methods of measuring

capacitance with the CTMU. The first is the absolute

method, in which the actual capacitance value is

desired. The second is the relative method, in which

the actual capacitance is not needed, rather an

indication of a change in capacitance is required.

19.4.1

ABSOLUTE CAPACITANCE

MEASUREMENT

For absolute capacitance measurements, both the

current and capacitance calibration steps found in

Section 19.3 “Calibrating the CTMU Module”

should be followed. Capacitance measurements are

then performed using the following steps:

Initialize the A/D Converter.

Initialize the CTMU.

Set EDG1STAT.

Wait for a fixed delay, T.

Clear EDG1STAT.

Perform an A/D conversion.

Calculate the total capacitance, CTOTAL = (I * T)/V,

where I is known from the current source

measurement step (see Section 19.3.1 “Current

Source Calibration”), T is a fixed delay and V is

measured by performing an A/D conversion.

Subtract the stray and A/D capacitance

(COFFSET from Section 19.3.2 “Capacitance

Calibration”) from CTOTAL to determine the

measured capacitance.

19.4.2

RELATIVE CHARGE

MEASUREMENT

An application may not require precise capacitance

measurements. For example, when detecting a valid

press of a capacitance-based switch, detecting a rela-

tive change of capacitance is of interest. In this type of

application, when the switch is open (or not touched),

the total capacitance is the capacitance of the combina-

tion of the board traces, the A/D Converter, etc. A larger

voltage will be measured by the A/D Converter. When

the switch is closed (or is touched), the total

capacitance is larger due to the addition of the

capacitance of the human body to the above listed

capacitances, and a smaller voltage will be measured

by the A/D Converter.

Detecting capacitance changes is easily accomplished

with the CTMU using these steps:

Initialize the A/D Converter and the CTMU.

Set EDG1STAT.

Wait for a fixed delay.

Clear EDG1STAT.

Perform an A/D conversion.

The voltage measured by performing the A/D

conversion is an indication of the relative capacitance.

Note that in this case, no calibration of the current

source or circuit capacitance measurement is needed.

See Example 19-4 for a sample software routine for a

capacitive touch switch.

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