[...] i`m wondering, if i realy need to connect external crystal to MCU ? Or it is needed only when i want to operate in higher frequency or in some specific frequency?
Some controllers need a crystal, others can (also) be run from an external RC oscillator with a timing accuracy of around 1%, if less accurate timing can be tolerated.
IOW If you're going to do anything time critical, like high speed comms, event timing or similar, you need a crystal. For most other jobs, the internal oscillator will be fine.
A 1% error is the equivalent of 14.4 minutes in 24 hours, so clearly the RC oscillator is a nogo for a time keeping job.
I have Atmega16 and Atmega8 wich is dead due fuse bit fail
How do you determine the cause to be brickin'?
Do you hope to bring them back?
If so, here's a couple of possibilities, but I'd personally use them as door stops and just get new ones, but of course, if you keep bricking them... If you've killed them in off any other way, there's no help in those.http://www.mtcnet.net/~henryvm/4AvrFuseBuster/http://www.der-hammer.info/hvprog/index_en.htm
My goal is to make a tank robot (have 4 360 deg. hacked servos) and in future tachometer for car.
You only need 1 servo/motor for a tank thread. With 2 motors, they will be battling each other if they're not exactly the same speed (and they never are).