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2010 Microchip Technology Inc. DS01171C-page 1
AN1171
INTRODUCTION
This application note describes the use of the
Capacitive Sensing Module (CSM) present on all
PIC16F72X devices. The CSM simplifies the amount of
hardware and software setup needed for capacitive
sensing applications. Only the sensing pads on the
PCB need to be added. It is recommended that
application note AN1101, “Introduction to Capacitive
Sensing” be read in order to understand the capacitive
sensing concepts.
CAPACITIVE SENSING MODULE
The CSM allows the user to design a capacitive
sensing system without an external oscillator circuit.
The CSM has its own software-controlled oscillator. It
can also monitor up to 16 inputs. In a typical
application, the CSM is directly attached to pads on a
PCB and covered by an insulating material. When the
insulating material above a pad is touched by the user’s
fingertip, the capacitance of the pad increases, thus
causing a frequency shift in the CSM. For more
information on the CSM hardware, please refer to the
device data sheet.
This module simplifies the software needed for
capacitive sensing: it is only necessary to initialize a
few registers and then set the appropriate method of
measuring the change in frequency.
MODULE INITIALIZATION
To initialize the CSM, the appropriate cap sense inputs
must be initialized as analog inputs. Then, the CSM
registers are set, as shown in Example 1.
EXAMPLE 1: SETUP OF THE CAPACITIVE SENSING MODULE
FREQUENCY MEASUREMENT
Once these registers are set, the module will start oscil-
lating. Now, the appropriate method of measuring the
frequency needs to be set. There are several methods
that can be applied:
• Use Timer0 as a timer resource for the CSM.
• Use Timer2 as a timer resource. Timer2 has a
greater flexibility in defining the time base by
using PR2 to set the desired time base.
• Use the WDT wake from Sleep event as the time
base.
FREQUENCY MEASUREMENT:
TIMER1 GATE
All of these methods use the Timer1 gate input. Timer1
will act as a counter; it will increment at every rising
edge of the Cap Sensing Module output frequency. The
time base selected will start and stop the counter. The
user can then read the value on Timer1, which would
be a measure of the oscillator frequency. It is recom-
mended to use Timer1 Gate in One-Shot mode to
measure the full-cycle length of the chosen time base.
Author: Enrique Aleman
Microchip Technology Inc.
TRISA = 0x30;
ANSELA = 0x30; // CPS7, CPS6 initialized as analog inputs
TRISB = 0x3F;
ANSELB = 0x3F; // CPS0-CPS5 initialized as analog inputs
TRISD = 0xFF;
ANSELD = 0xFF; // CPS8-CPS15 initialized as analog inputs
CPSCON0 = 0x8C; // Cap sense on, high range oscillator,
CPSCON1 = 0x00; // Cap sense channel input 0 is selected
How To Use The Capacitive Sensing Module (CSM)
