Datasheet

 2010 Microchip Technology Inc. Preliminary DS39974A-page 405
PIC18F47J13 FAMILY
26.4 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.
26.4.1 ABSOLUTE CAPACITANCE
MEASUREMENT
For absolute capacitance measurements, both the
current and capacitance calibration steps found in
Section 26.3 “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, C
TOTAL = (I * T)/V,
where I is known from the current source
measurement step (see Section 26.3.1 “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
(C
OFFSET from Section 26.3.2 “Capacitance
Calibration”) from C
TOTAL to determine the
measured capacitance.
26.4.2 CAPACITIVE TOUCH SENSE USING
RELATIVE CHARGE
MEASUREMENT
An application may not require precise capacitance
measurements. For example, when detecting a valid
press of a capacitance-based touch sense button,
detecting a relative change of capacitance is of inter-
est. In this type of application, when the touch sense
pad is not being pressed, 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 touch sense pad is
pressed, the total capacitance is larger due to the addi-
tion of the capacitance of the human body, therefore, 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 26-4 for a sample software routine for a
capacitive touch switch.