# 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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PIC24F Family Reference Manual
DS39724B-page 11-10 © 2010 Microchip Technology Inc.
11.3 CTMU OPERATION
The CTMU works by using a fixed current source to charge a circuit. The type of circuit depends
on the type of measurement being made. In the case of charge measurement, the current is fixed
and the amount of time the current is applied to the circuit is fixed. The amount of voltage read
by the A/D is then a measurement of the capacitance of the circuit. In the case of time
measurement, the current, as well as the capacitance of the circuit, is fixed. In this case, the
voltage read by the A/D is then representative of the amount of time elapsed from the time the
current source starts and stops charging the circuit.
If the CTMU is being used as a time delay, both capacitance and current source are fixed, as well
as the voltage supplied to the comparator circuit. The delay of a signal is determined by the
amount of time it takes the voltage to charge to the comparator threshold voltage.
11.3.1 Theory of Operation
The operation of the CTMU is based on the equation for charge, as shown in Equation 1.
Equation 11-1:
More simply, the amount of charge measured in coulombs in a circuit is defined as current in
amperes (I) multiplied by the amount of time in seconds that the current flows (t). Charge is also
defined as the capacitance in farads (C) multiplied by the voltage of the circuit (V), as shown in
Equation 11-2.
Equation 11-2:
The CTMU module provides a constant, known current source. The A/D Converter is used to
measure (V) in the equation, leaving two unknowns: capacitance (C) and time (t). Equation 11-2
can be used to calculate capacitance or time, by either the relationship shown in Equation 11-3
and using the known fixed capacitance of the circuit, or by Equation 11-4 using a fixed time that
the current source is applied to the circuit.
Equation 11-3:
Equation 11-4:
I C
dV
dT
-------=
I t C V=
t
C V 
I
-----------------=
C
I t 
V
-------------=

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