From a block diagram point of view, the A3987 is indeed a self-contained motor driver with logic, current control, ancillary, and dual H-bridge sections. The logic section allows the user to set the step function, MS1 and MS2, for full, half, quarter, and sixteenth step selection, direction of rotation, sleep mode, and an enable mode. The logic section handles the translation of step information and uses the ltrip (max) level, set by the current control section, to determine current levels each H-bridge will be set at to properly position the rotor for the micro-step needed. This section also provides, through internal D to A’s, the proper signal to set the current level in the current control section.