Dear Reader,

I may sound little naive but actually it confuses me a lot, whenever I select a MCU either from TI or ATMEL, their apart from 32.768KHz crystal we need to select another crystal either of 4MHz, 12MHz, 16MHz etc. Please anyone explain what are the factors that impact on the crystal frequency selection of MCU's and Processors also. Usually datasheet reflects that a range of crystal frequency it supports. For example I am attaching datasheet of MSP430G2955 below
http://www.ti.com/lit/ds/symlink/msp430g2955.pdf

Best Regards
-Anand