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mTouch? CAPACITIVE EVALUATION
KIT USER’S GUIDE
Chapter 2. The Demonstration Application
This chapter describes the touch sense application that is preprogrammed on the
PIC24F and PIC16F microcontrollers, and its general principles of operation. Topics
included are:
2.1
INTRODUCTION TO THE TOUCH INTERFACE
On the PIC16F727 Motherboard, the on-chip Cap Sense Module (CSM) creates a
relaxation oscillator to perform touch sensing. The period or frequency of the relaxation
oscillator can be measured, and when the sensor is touched, the frequency will drop
and the period will increase, indicating a touched condition.
Control of the touch sense features is built on the PIC24F microcontroller ’s on-chip
Charge Time Measurement Unit (CTMU) module. The CTMU consists of a constant
current source that charges each touch circuit to a fixed voltage level. When any addi-
tional capacitance is added to the circuit (from the touch of a fingertip, for example), the
fixed current source will now charge the circuit to a lower voltage. This change is how
the microcontroller detects a touch event. A more detailed description of the CTMU’s
operation is provided in Section 11, “Charge Time Measurement Unit (CTMU)”
(DS39724) of the “ PIC24F Family Reference Manual” .
The microcontrollers use the CTMU or CSM to monitor its analog input channels, which
are in turn connected to capacitive touch pad sensors on the top layer of the circuit
board.
The evaluation board has four different sensor demonstration boards:
1.
2.
3.
4.
Direct 8 Key Plug-in Board
Matrix Key Plug-in Board
2-Channel Slider Plug-in Board
4-Channel Slider Plug-in Board
A more detailed description of the CAP TOUCH – CTMU Evaluation Board’s operation
2.1.1
Touch Sensitivity
The response of the sensor to fingertip touch is influenced by many factors: touch
areas, voltage and current levels, ambient humidity, static buildup, and so on. Most of
these factors have been accounted for in designing the demo application firmware, and
are based on typical environmental values, and certain assumed constants. The demo
application is very flexible in the sense that it can be modified by the user.
The PC side application accompanying the mTouch Capacitive Evaluation Kit can be
used to change the sensitivity of the sensors by writing trip point information back
? 2009 Microchip Technology Inc.
DS41385A-page 11
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