PIC18F87J90 Datasheet, PDF (317/450 Pages) Microchip Technology – 64/80-Pin, High-Performance Microcontrollers with LCD Driver and nanoWatt Technology

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24.4 Measuring Capacitance with the
CTMU
There are two separate methods of measuring capaci-
tance 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.
24.4.1
ABSOLUTE CAPACITANCE
MEASUREMENT
For absolute capacitance measurements, both the
current and capacitance calibration steps found in
Section 24.3 “Calibrating the CTMU Module” should
be followed. Capacitance measurements are then
performed using the following steps:
1. Initialize the A/D Converter.
2. Initialize the CTMU.
3. Set EDG1STAT.
4. Wait for a fixed delay, T.
5. Clear EDG1STAT.
6. Perform an A/D conversion.
7. Calculate the total capacitance, CTOTAL = (I * T)/V,
where I is known from the current source
measurement step (Section 24.3.1 “Current
Source Calibration”), T is a fixed delay and V is
measured by performing an A/D conversion.
8. Subtract the stray and A/D capacitance
(COFFSET from Section 24.3.2 “Capacitance
Calibration”) from CTOTAL to determine the
measured capacitance.
24.4.2
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 relative
change of capacitance is of interest. In this type of appli-
cation, when the switch is open (or not touched), the total
capacitance is the capacitance of the combination 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:
1. Initialize the A/D Converter and the CTMU.
2. Set EDG1STAT.
3. Wait for a fixed delay.
4. Clear EDG1STAT.
5. Perform an A/D conversion.
The voltage measured by performing the A/D conver-
sion 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 24-4 for a sample software routine for a
capacitive touch switch.
 2010 Microchip Technology Inc.
DS39933D-page 317