PIC18F26K80-I Datasheet, PDF (244/622 Pages) Microchip Technology – 28/40/44/64-Pin, Enhanced Flash Microcontrollers with ECAN™ and nanoWatt XLP Technology

PIC18F66K80 FAMILY
18.4.2 CAPACITANCE CALIBRATION
There is a small amount of capacitance from the inter-
nal A/D Converter sample capacitor as well as stray
capacitance from the circuit board traces and pads that
affect the precision of capacitance measurements. A
measurement of the stray capacitance can be taken by
making sure the desired capacitance to be measured
has been removed.
After removing the capacitance to be measured:
1. Initialize the A/D Converter and the CTMU.
2. Set EDG1STAT (= 1).
3. Wait for a fixed delay of time, t.
4. Clear EDG1STAT.
5. Perform an A/D conversion.
6. Calculate the stray and A/D sample capacitances:
COFFSET = CSTRAY + CAD = (I • t)/V
Where:
• I is known from the current source measurement
step
• t is a fixed delay
• V is measured by performing an A/D conversion
This measured value is then stored and used for
calculations of time measurement or subtracted for
capacitance measurement. For calibration, it is
expected that the capacitance of CSTRAY + CAD is
approximately known; CAD is approximately 4 pF.
An iterative process may be required to adjust the time,
t, that the circuit is charged to obtain a reasonable volt-
age reading from the A/D Converter. The value of t may
be determined by setting COFFSET to a theoretical value
and solving for t. For example, if CSTRAY is theoretically
calculated to be 11 pF, and V is expected to be 70% of
VDD or 2.31V, t would be:
(4 pF + 11 pF) • 2.31V/0.55 A
or 63 s.
See Example 18-3 for a typical routine for CTMU
capacitance calibration.
DS39977F-page 244
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