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On the DRV8825EVM, the PPS stands for Pulses Per Second. Notice this is not the same as steps per second (SPS) as depending on the microstepping resolution a pulse may or may not be a step. If you have full step commutation and 1000 PPS, then you move at 1000 Steps per second. But if you are on Half Step, then you are moving at 500 steps per second.
The acceleration rate and time base are just as described on the picture. The acceleration rate is how many clock cycles is the timer increased/decreased on a per time base basis, and the time base is how many milliseconds pass between speed change internvals. This has to do with how the motor is accelerated/decelerated.
There is one point, however, of which I am not too proud. The PPS is in pulses per second, but the acceleration rate is in internal timer clock cycles. Each clock cycles is 125 ns (1/8MHz). To know how much a PPS represents in clock cycles we need to divide 4,000,000 by the PPS. For example, to generate 1000 PPS, you would need 4000 timer clock cycles. The MSP430 hardware uses this number inside one of the Timer Compare modules and generates the pulse which is fed into the STEP input.
When the motor is started, the starting speed is always 0xFFFF or 65535 clock cycles (same as 61 PPS). The firmware will then decrement the number of clock cycles per step according to the acceleration rate and the time base. For example, if the acceleration rate is 100 and the time base is 1 millisecond, then each millisecond the internal timer is decremented by a 100. You can then imagine that at first the steps will be generated at 65535 clock cycles per step (61 PPS), 1 ms after they will be generated at 64535 clock cycles per step and so on until the desired speed is reached.
I realize this convoluted method is not the easiest to understand and is not the best either. We are writing an application note which denotes the best way to do this which is to use an acceleration equal to a number in Steps Per Second Per Second. But explaining this method is quite convoluted itself so once the app note is released, let us know which parts are not easily understood.
