Based on the Motorola (now Freescale) heavily used but obsolete SAA1042 stepper-motor-driver IC, this Design Idea describes a CPLD (complex-programmable-logic-device)-based implementation of a stepper-motor driver that can also replace the driver in SAA1027- or UCN5804B-based designs.  The design uses only six macrocells of a Xilinx XC9536 CPLD and thus can implement multiple stepper-motor drivers in one small-capacity CPLD.

The CPLD stepper-motor driver requires clock, direction, step-size, and reset inputs.The clock input accepts logic-level pulses and goes active on the pulse's positive edge.The direction, or CW/CCW (clockwise/counterclockwise), input determines the motor's rotational direction. Depending on the motor's electrical connections, holding this input at 0V normally produces CW rotation, and a logic-1 input produces CCW rotation.

The step-size—that is, full- or half-step—input determines the motor's angular rotation for each clock pulse. Holding this input low commands the motor to execute a full step for each applied clock pulse, and a high input produces a half-step. A high level on the reset input puts the motor in a previously defined state and commands the CPLD to ignore any incoming clock pulses.

The CPLD's outputs comprise A and A_n and B and B_n phases, each of which controls one of the motor's two coils through external power drivers IC 2 and IC 3 , which operate at the motor's nominal voltage. A pair of Schottky diodes at each driver's output protects the drivers' outputs during inductive-voltage transients induced by reversing the windings' currents. Using MOSFET drivers with internal diodes, such as Microchip's TC4424A dual driver, may eliminate the requirement for external diodes.