Most of the 8-bit microcontrollers lack integrated DAC (Digital to Analog Converter) functionality. This is handy when you need to generate analog signals out of digital information. Adding DAC to any existing microcontroller is a piece of cake. But before you start why not to look at various options available. Embedded newbie provides a review of Arduino DAC solutions. Beginning with the R-2R ladder solution list goes through multiple ways of converting digital to analog.
Depending on your needs and speed required there can be a PWM DAC converter that is nothing more than digital signal passed through low pass filter. This is how motor control works. Its relatively slow, but serial that gives an advantage when small pin count microcontrollers are used. In the other hand is signal speed is an issue then parallel DAC – same R-2R ladder probably with an output buffer circuit. And lastly, there is always an option to use specialized DAC chips that can be interfaced through one of the available interfaces like SPI. These save space and MCU pin and still provide high resolution and speed.