AN1250 Datasheet, PDF(1/22 Page) Microchip Technology – Microchip CTMU for Capacitive Touch Applications

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AN1250 Datasheet, PDF (1/22 Pages) Microchip Technology – Microchip CTMU for Capacitive Touch Applications

AN1250

Microchip CTMU for Capacitive Touch Applications

Author: Bruce Bohn

Microchip Technology Inc.

INTRODUCTION

This application note describes the use of Microchipâs

Charge Time Measurement Unit (CTMU) for capacitive

touch applications. The CTMU is an excellent periph-

eral for use in touch sensing applications. The following

are some of the benefits of using the CTMU for touch

sensing applications:

â¢ Easy hardware setup

â¢ Simple to use software algorithms, available free

of charge

â¢ High speed allows for greater scan rate of

capacitive touch switches and for many other

processor tasks

â¢ Low cost and low component count; no external

hardware other than a connection to the copper

sensor pad

The purpose of this application note is to inform

prospective users of the CTMU for capacitive touch on

general usage guidelines, CTMU setup and software

algorithms, hardware and layout considerations, and

advanced capacitive touch applications, such as matrix

keys, sliders and multiple press keys.

The CTMU is currently available on selected PIC24

and PIC18F parts. Wherever possible, sample code for

both has been included.

All hardware examples used in this application note

directly pertain to the PICDEMâ¢ Touch Sense 2

Demonstration Board (Part Number DM164128). The

PICDEM Touch Sense 2 Demo Board uses a PIC24F

device-based microcontroller (PIC24FJ256GB110).

This demo board is not necessary for understanding this

application note.

GENERAL FEATURES OF THE CTMU

The CTMU is made up of a constant current source and

several logic blocks for it to operate:

â¢ The constant current source is connected to the

A/D converter of the microcontroller.

â¢ A switch connected to the CTMU constant current

source (and the A/D converter) is provided to

allow accumulated charge to be drained.

â¢ Two external pins are provided to trigger the

constant current source.

â¢ An output pin is also available for CTMU use.

For the purposes of capacitive touch, the external

pins for triggering the CTMU and the CTMU output

pin are not used.

The connection of the CTMU to the A/D converter is all

that is needed for capacitive touch applications. For a

more in-depth discussion on the CTMU module and

how it is used to measure charge or time, refer to the

CTMU chapter of the âPIC24F Family Reference Man-

ualâ for PIC24F microcontrollers, or the appropriate

data sheet for PIC18F microcontrollers on the

Microchip web site (http://www.microchip.com).

See Figure 1 for a block diagram of the CTMU hardware.

Note that there are a few slight differences in the CTMU

module from the PIC18F and the PIC24F. The PIC18F

CTMU module does not have an automatic ADC

conversion trigger. This is of no consequence for

capacitive touch applications. For clarity, the automatic

ADC trigger present on the PIC24F has not been used

in the sample code so that it more closely matches the

PIC18F code.

Also note that the CTMU on the PIC18F has different

internal trigger sources. (ECCP1 and ECCP2 for

PIC18F and Timer1 and OC1 for PIC24F). This is

mentioned purely for reference, and again, makes no

difference for capacitive touch applications. The sample

code listings in this application note manually set/clear

the EDG1STAT1 and/or EDG2STAT bits in the CTMU

control register for starting and stopping the CTMU

current source.

DS01250A-page 1

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