Aplicaciones Pic Con Ctmu

AN1375
See What You Can Do with the CTMU
Authors:

ABOUT THE CTMU

Padmaraja Yedamale and Jim Bartling
Microchip Technology Inc.

In a nutshell, the CTMU is an on-chip constant-current
source, surrounded by digital circuitry to precisely
control its operation (Figure 1). The current source
operates over four decade ranges, from 0.55 µA to
550 µA. When combined with the on-chip A/DConverter and comparators, the CTMU can perform a
variety of basic functions:

INTRODUCTION
Since its introduction in PIC® microcontrollers, the
Charge Time Measurement Unit (CTMU) has become
popular for creating simple and low component touch
control solutions. Some applications have made use of
its ability to resolve the time difference between inputs
down to the sub-nanosecond range.But thinking that
the CTMU can only deal with time and charge
measurements would be a serious underestimation of
its abilities.

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While the basic functions are useful for a variety of
applications, they can also be used as the basis of
more complex applications, such as:

As proof of its versatility, this application note provides
48 different applications (or, in a few cases,categories
of applications) that can be implemented with the
CTMU. Many of these implement new functionality in
existing control applications, using only a few or no
additional components. Keep in mind that these applications are basic ideas, presented in an abbreviated
format. Microchip may not offer reference designs or
source code for some applications. The reader is
invited to usethese ideas as the starting point for
developing their own solutions.

FIGURE 1:

Capacitance measurement (relative and absolute)
Inductance measurement (relative)
Resistance measurement (relative and absolute)
High-resolution time measurement

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Temperature measurement
Current source (constant and variable)
Precise time delay generation
Pulse-Width Modulation (PWM) outputThe different types of applications are covered in the
sections that follow.

CTMU BLOCK DIAGRAM

CTMUCON
CTMUICON

Current Source

CTED1
CTED2

Edge
Control
Logic

Current
Control

Timer1

CTMU
Control
Logic

Pulse
Generator

OC1
A/D Converter

A/D Trigger

CTPLS

Comparator 2
Input
Comparator 2 Output

 2011 Microchip Technology Inc.

DS01375A-page 1AN1375
APPLICATIONS BASED ON
ELECTRICAL PRINCIPLES

2.

The capacitance of the microphone’s element changes
continuously in proportion to the frequency of the vibrations hitting its diaphragm. The microcontroller’s A/D
constantly samples the resulting voltage and creates a
digital signal.

Relative Capacitance Measurement
By far, relative capacitance measurement is the most
usedprinciple in designing applications for the CTMU.
This is not surprising, because there are numerous
applications that require relative capacitance
measurement.

3.

4.

Stud Finder

A stud on the other side of the wall (metal or not, with
or without nails or metallic fasteners) will change the
local capacitance of the wall’s surface.

As an example, take a simple touch application(shown
in Figure 2) with a total capacitance (including parasitics, like the switch (CSW) and circuit (CCIR) of 30 pF.
When the external circuit is charged with a current of
5.5 µA for 10 µs, this produces a voltage of 1.83V.
When you add the touch of a finger, an additional
capacitance (CF) of up to 10 pF is added. The exact
amount of capacitance depends on how much the
touch pad is coveredby the finger and any covering
material over the pad. For a 10 pF change, with the
same current and charge time, the voltage is 1.38V.

5.

Occupancy Sensing

Rather than using the old interrupted photocell principle,
a capacitance sensor can be embedded in the doorway.
Whenever a person passes through, the sensor’s
capacitance changes.
6.

Liquid Level Sensing

Here is a clever...