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AN1250 Datasheet(PDF) 1 Page - Microchip Technology |
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AN1250 Datasheet(HTML) 1 Page - Microchip Technology |
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1 / 22 page  © 2009 Microchip Technology Inc. DS01250A-page 1 AN1250 INTRODUCTION This application note describes the use of Microchip’s 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: • Easy hardware setup • Simple to use software algorithms, available free of charge • High speed allows for greater scan rate of capacitive touch switches and for many other processor tasks • 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 PICDEM™ 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: • The constant current source is connected to the A/D converter of the microcontroller. • A switch connected to the CTMU constant current source (and the A/D converter) is provided to allow accumulated charge to be drained. • Two external pins are provided to trigger the constant current source. • 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 “PIC24F Family Reference Man- ual” 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. Author: Bruce Bohn Microchip Technology Inc. Microchip CTMU for Capacitive Touch Applications |
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