Looking for a poor-man’s capacitive touch sensor using only the ADC on your MCU and a couple discrete components? NXP shows how it can be done in AN11023: Capacitive Touch Sensing using the LPC11xx (here for .zip file with code). (Note: While this app note is aimed at the ARM Cortex M0 LPC11xx series of chips, the information is easily transferable to any other MCU.)
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Microchip have a special peripheral in a number of their Microcontrollers called the Charge Time Measurement Unit (CTMU). You can use this peripheral for capacitive touch sensing, and a range of other applications:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en553967
Microchip also published an App. Note “Capacitive Touch Using Only an ADC (CVD)” with a similar idea.
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en545264
http://ww1.microchip.com/downloads/en/AppNotes/01298A.pdf
http://ww1.microchip.com/downloads/en/AppNotes/1298source.zip
You do not need an internal ADC in your micro-controller to do capacitive touch sensing. You can do it with general purpose digital I/O using the delta-sigma method. The load on your micro-controller can be significantly reduced if you employ your on-chip timer/counters. Do a Web search, there are many examples…