Microchip Technology DM163025-1 数据表

DS30684A-page 324

 2012 Microchip Technology Inc.

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 measure-
ment of the capacitance of the circuit. In the case of
time measurement, the current, as well as the capaci-
tance 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 capaci-
tance 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.

20.1.1

THEORY OF OPERATION

The operation of the CTMU is based on the following
equation for charge: 

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). It follows that: 

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). The above equation can be used to calcu-
late capacitance or time, by either the relationship
using the known fixed capacitance of the circuit:

or by:

using a fixed time that the current source is applied to
the circuit.

20.1.2

CURRENT SOURCE

At the heart of the CTMU is a precision current source,
designed to provide a constant reference for measure-
ments. The level of current is user selectable across
three ranges or a total of two orders of magnitude, with
the ability to trim the output in ±2% increments
(nominal). The current range is selected by the
IRNG<1:0> bits (CTMUICON<1:0>), with a value of
‘01’ representing the lowest range. 
Current trim is provided by the ITRIM<5:0> bits
(CTMUICON<7:2>). These six bits allow trimming of
the current source in steps of approximately 2% per
step. Note that half of the range adjusts the current
source positively and the other half reduces the current
source. A value of ‘000000’ is the neutral position (no
change). A value of ‘100001’ is the maximum negative
adjustment (approximately -62%) and ‘011111’ is the
maximum positive adjustment (approximately +62%).

20.1.3

EDGE SELECTION AND CONTROL

CTMU measurements are controlled by edge events
occurring on the module’s two input channels. Each
channel, referred to as Edge 1 and Edge 2, can be con-
figured to receive input pulses from one of the edge
input pins (CTED1 and CTED2), Timer1 or Output
Compare Module 1. The input channels are level-
sensitive, responding to the instantaneous level on the
channel rather than a transition between levels. The
inputs are selected using the EDG1SEL and EDG2SEL
bit pairs (CTMUCONL<3:2> and <6:5>). 
In addition to source, each channel can be configured for
event polarity using the EDGE2POL and EDGE1POL
bits (CTMUCONL<7,4>). The input channels can also
be filtered for an edge event sequence (Edge 1 occur-
ring before Edge 2) by setting the EDGSEQEN bit
(CTMUCONH<2>).

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C V.

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 I

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 V

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