PIC18F25J11-I/PT MICROCHIP [Microchip Technology], PIC18F25J11-I/PT Datasheet - Page 381

• Manufacturer Part Number: PIC18F25J11-I/PT
• Description: 28/44-Pin, Low-Power, High-Performance Microcontrollers with nanoWatt XLP Technology
• Manufacturer: MICROCHIP [Microchip Technology]

24.4
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
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.
2.
3.
4.
5.
6.
7.
8.
© 2009 Microchip Technology Inc.
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, C
where I is known from the current source
measurement step (see Section 24.3.1 “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
(C
Calibration”) from C
measured capacitance.
OFFSET
Measuring Capacitance with the
CTMU
ABSOLUTE CAPACITANCE
MEASUREMENT
from Section 24.3.2 “Capacitance
TOTAL
to determine the
TOTAL
= (I * T)/V,
PIC18F46J11 FAMILY
24.4.2
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:
1.
2.
3.
4.
5.
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.
Initialize the A/D Converter and the CTMU.
Set EDG1STAT.
Wait for a fixed delay.
Clear EDG1STAT.
Perform an A/D conversion.
RELATIVE CHARGE
MEASUREMENT
DS39932C-page 381