AC323027

AN1250 Microchip CTMU for Capacitive Touch Applications GENERAL FEATURES OF THE CTMU Author: Bruce Bohn Microchip Technology Inc. The CTMU is made up of a constant current source and several logic blocks for it to operate: INTRODUCTION The constant current source is connected to the A/D converter of the microcontroller. This application note describes the use of Microchips A switch connected to the CTMU constant current Charge Time Measurement Unit (CTMU) for capacitive source (and the A/D converter) is provided to touch applications. The CTMU is an excellent periph- allow accumulated charge to be drained. eral for use in touch sensing applications. The following Two external pins are provided to trigger the are some of the benefits of using the CTMU for touch constant current source. sensing applications: An output pin is also available for CTMU use. Easy hardware setup For the purposes of capacitive touch, the external Simple to use software algorithms, available free pins for triggering the CTMU and the CTMU output of charge pin are not used. High speed allows for greater scan rate of The connection of the CTMU to the A/D converter is all capacitive touch switches and for many other that is needed for capacitive touch applications. For a processor tasks more in-depth discussion on the CTMU module and Low cost and low component count no external how it is used to measure charge or time, refer to the hardware other than a connection to the copper CTMU chapter of the PIC24F Family Reference Man- sensor pad ual for PIC24F microcontrollers, or the appropriate The purpose of this application note is to inform data sheet for PIC18F microcontrollers on the prospective users of the CTMU for capacitive touch on Microchip web site (AN1250 FIGURE 1: CTMU BLOCK DIAGRAM CTMUCON (CTMUCONH and CTMUCONL for PIC18F) CTMUICON Current Source External Edge Trigger Pins CTMU A/D Conversion Edge Control Trigger Control Logic (PIC24F only) Logic Current Control Timer1 (PIC24F only) Pulse Pulse Output Generation OC1 Pin (PIC24F only) Logic ECCP1/ECCP2 A/D Converter Comparator 2 Input (PIC18F only) Comparator 2 Output DS01250A-page 2 2009 Microchip Technology Inc.