scorp84:
I want to ask that how can I use external clock with my arduino since I need a clock much higher than 16Mhz and I saw in the datasheet that one can use external clock by doing pre scaling so I want to ask that it it possible to use a clock much higher than 16 MHz and if someone can suggest that how I can do it?
How high is "much" higher? I think you are cross-posting:
http://arduino.cc/forum/index.php/topic,98482.0/topicseen.html
Rather than float multiple potential solutions, and ask how to make them work, how about giving us a nice concise problem statement so we can discuss what a possible solution might look like. If you read the datasheet, then you saw that 20MHz is the maximum system clock frequency. In my book, that's not "much" higher.
If I wanted to measure an 8µs pulse, I might look to the Timer/Counter1 Input Capture Unit. With the timer running at system clock speed (prescaler = 1), that gives a 62.5ns resolution, so a measurement within a couple percent ought to be possible. If orders of magnitude more precision is needed, then I'd look to other hardware.