DS18B20 temp sensor IC
DS18B20 is a digital Thermometer, thus you will not need any ADC. DS18B20 communicates with microcontroller through 1-wire protocol. You can also implement 1-wire protocol using GPIO. see below link for more details on software implementation of 1-wire protocol
http://www.maximintegrated.com/en/app-notes/index.mvp/id/126