Some MCU have QTouch support - # touch channels implemented as a hardware. Is it true that QTouch library was designed only to work with such MCUs? Can I use it with, for example, 'old' ATmega48? Is it only a software solution which can be ported to any AVR microcontroller despite of hardware QTouch support?
You can use QTouch also on MCUs which do not have dedicated touch controller peripheral attached to it. Although there will be some limitations like only one single sensing method can run at a time. Also, it will consume more CPU time and more power.
Check out Atmel's site: http://www.atmel.com/products/touchsolutions/bsw/touch-support-for-mcus.aspx
For devices without peripheral touch controller, Atmel offer a software based solution that allow high degree of freedom for microcontroller selection and configuration. Most Atmel AVR and SMART ARM devices are supported and the library provides robust and industry proven capacitive touch performance. QTouch library supports both self and mutual capacitance sensing. Unlike microcontrollers with hardware touch module (PTC) the software library is only capable of running a single sensing method at a time. A pure software based touch system will also use more power and CPU time than a PTC based system. Code development and debugging is made simple by QTouch composer which is a free extension to the Atmel Studio IDE software.