David rob wrote:
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want to do is i need to keep counting the steps generated by the stepper motor from an initial position and need to transmit the step data through serial port after a certain time.


Your stepper motor does not generate step data...
Your ISR should make your stepper motor move, and keep track of the step count...



i dont want my motor to stop at any time.it shld go for 360 deg. and then come back. and keep sweeping as the data is being send thro' UART
i would like to know if i could use an angle less than 1.8.my step angle is 1.8 and the half-stepping will give me 0.9 but i want to try and make it even less.


If you do not want your motor to stop, then you just do not stop your timer that generates the interrupts for you ISR. And as your ISR is driving your stepper motor..

But bear in mind that your stepper does not really continuously rotate, it makes "discrete steps" each time you "step" it... ;-)


regards
Patrick