Interfacing PIC Microcontrollers 22. Explore in detail microcontroller interfacing techniques using the popular PIC 16F877 Work through step-by-step examples interactively using circuit simulation software, supplied as assembly source code Gain the knowledge of a wide range of peripheral devices such as keyboards, displays, sensors and drives and serial communication with other processors, memory and more Use interactive simulation software to design and test circuits. | Interfacing PIC Microcontrollers b EN IN1 IN2 S1 S2 S3 S4 Motor 0 x x off off off off off 1 0 0 off ON off ON off 1 1 0 ON off off ON FORWARD 1 0 1 off ON ON off REVERSE 1 1 1 ON off ON off off Figure Full bridge driver a block diagram b function table In Figure a the stepper motor stator has 16 poles and the rotor 4 north poles which are attracted to the active pairs of rotor south poles. There are four sets of windings A B C D connected sequence around the stator. These windings have their other ends connected to common terminals. This gives a total of six connections to the motor with two pairs of centretapped windings. This allows the motor to be driven in different modes while keeping the number of connections to a minimum. In the test circuit the common terminals are connected to the power supply 12 V and the individual coil terminals driven from the sequencer active low Figure . In normal full-step mode the coil sets are activated in pairs Figure b and the rotor moves half a pole per step giving 24 steps per revolution. The step size is then 360 24 15 . This mode provides full torque but lower positional resolution. In half-step mode the rotor moves by a quarter pole per step providing twice as many steps per revolution but less torque since only one coil pair is activated at a time. There are two chips forming the stepper drive interface. The L297 controller provides the stepping sequence on outputs A B C and D and the L298 full bridge driver provides the drive current needed by the motor windings. The drive mode full or half step is fixed in full step by tying the step mode select input low. The active low reset is tied high. The MCU provides an enable signal and selects the direction of rotation clockwise CW or counter-clockwise 196 Power Outputs Rotor Figure Stepper motor operation a windings sequence b drive sequence normal mode CCW and the test program outputs clock pulses at a frequency of 20 Hz so that the stepping effect .
