Daniel,

I can't offer you code, but I can offer you advice. Use a timer in 16-bit mode with a variable preload to vary the interval of overflow. Enable the timer's interrupt. In this timer's interrupt service routine place a state machine that sends the proper control signals to the stepper motor. The state machine could be controlled from the main thread by a couple of bits, ex. direction,half/full step, some ints, ex. number of steps to take. The value of the preload would determine the speed of the motor. 12MHz, Ex, 0000h =~15.3 steps per second, FF00h =~3921.6 steps per second. 24MHz clock would double these values, etc. Other schemes may offer better resolution/control.
If better performance is desired, I would use a microcontroller with a more efficient architecture. A traditional 8051 may not be fast enough to get out of its own way at high motor speeds in this example. Surely someone has done this. Anyone?

Cory Spackman