PIC18F25K80 MICROCHIP [Microchip Technology], PIC18F25K80 Datasheet - Page 252

• Manufacturer Part Number: PIC18F25K80
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F66K80 FAMILY
18.5
There are two ways to measure capacitance with the
CTMU. The absolute method measures the actual
capacitance value. The relative method only measures
for any change in the capacitance.
18.5.1
For absolute capacitance measurements, both the
current and capacitance calibration steps found in
Section 18.4 “Calibrating the CTMU Module” should
be followed.
To perform these measurements:
1.
2.
3.
4.
5.
6.
7.
8.
DS39977C-page 252
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
• T is a fixed delay
• V is measured by performing an A/D conversion
Subtract the stray and A/D capacitance
( C
Calibration” ) from C
measured capacitance.
measurement step ( Section 18.4.1 “Current
Source Calibration” )
OFFSET
Measuring Capacitance with the
CTMU
ABSOLUTE CAPACITANCE
MEASUREMENT
from Section 18.4.2 “Capacitance
TOTAL
to determine the
TOTAL
= (I * T)/V ,
Preliminary
measurements. When detecting a valid press of a
18.5.2
Not all applications require precise capacitance
capacitance-based switch, only a relative change of
capacitance needs to be detected.
In such an application, 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 and
other elements. A larger voltage will be measured by the
A/D Converter. When the switch is closed (or 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.
To detect capacitance changes simply:
1.
2.
3.
4.
5.
The voltage measured by performing the A/D conver-
sion is an indication of the relative capacitance. In this
case, no calibration of the current source or circuit
capacitance measurement is needed. (For a sample
software routine for a capacitive touch switch, see
Example
Initialize the A/D Converter and the CTMU.
Set EDG1STAT.
Wait for a fixed delay.
Clear EDG1STAT.
Perform an A/D conversion.
18-4.)
CAPACITIVE TOUCH SENSE USING
RELATIVE CHARGE
MEASUREMENT
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