What is a Timer ? 

A Timer is device it sense the time interval and produce an output at a set value . example alarm Timer setting.(Timer in Wikipedia )

,What is the need of a timer in Micro controller ?
1) Some time  micro controller Code need some accurate  Delay  ( Example blinking An LED in each      accurate 1 sec. Look LED Blinking example

2)Some Time the code needs the code Repeat

3) Most of the Interfacing devices with micro-controller needs a clock (accurate interval of clocks)
   Example:   LCD module require Clock  Signal Look LCD Interfacing

Timer in 8051

Also a Timer actually require a clock pulse  (pling) .These pulse are given from controller  (actually it is in software based not giving ) see below picture
8051 type controller has  two Timer Timer 0 and Timer 1 (These two timer is the length of 16 bit ,0000 to  FFFF ).These Timer is named as TR0 and TR1. 

The Timer Setting/configuring  can controlling by the TMOD register 

These TMOD register Values is used to decide how the Timer Works .
Sample program

download the fullcode and Test

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