# Vault 7: Projects

This publication series is about specific projects related to the Vault 7 main publication.

### Protego

# PIC24FJ32MC10X -- Charge Time Measurement Unit

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© 2010 Microchip Technology Inc. DS39724B-page 11-21
Section 11. CTMU
CTMU
11
11.6 MEASURING CAPACITANCE WITH THE CTMU
There are two separate methods of measuring capacitance 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.
11.6.1 Absolute Capacitance Measurement
For absolute capacitance measurements, both the current and capacitance calibration steps
found in Section 11.5 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 (Section 11.5.1 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 11.5.2 Capacitance
Calibration) from C
TOTAL to determine the measured capacitance.
11.6.2 Relative Charge Measurement
An application may not require precise capacitance measurements. For example, when detect-
ing a valid press of a capacitance-based switch, detecting a relative 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 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 conversion is an indication of the relative
capacitance. Note that in this case, no calibration of the current source or circuit capacitance
measurement is needed. A sample software routine for a capacitive touch switch is shown in
Example 11-7.

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