How can I choose a microcontroller that fits my needs?

Actually there is no specific way to do that but some considerations must be taken when we choose our microcontroller.

• The size of the program memory for example if i want to design a LED blinking system that needs at most 1 k words of program memory i will not use PIC18F452 for example that provides 16 K of program memory. I will use for example 12Fxx or 16F630....
• The number of input/output used in the project so i will choose a microcontroller that fits that purposes for example if i want to make a timer for a serve time relay with one switch or 2 as 2 inputs and one or 2 ouputs for this timer so as i will use one port with 8 bit .... No needs of huge number of I/O s .
• Additional integrated modules that costs me additional fees and i do not use it....for example :A/D converter,Comparators,multiple timers,SPI,USB port,........ So if my project is a pure digital system that do not need analog signal ;no need for A/D converter which is expensive .
• Finally The speed my applications needs for example all the PIC16FXX had a maximum crystals frequency of 20 MHZ and there are applications that needs more than that frequency so we will use the PIC18FXX family instead.