Embedded Systems Design with the Atmel AVR Microcontroller Part II

to PD4
470
200
7.6. DC MOTOR SPEED AND DIRECTION CONTROL 221
ZTX451
ZTX551
+
12 VDC
M
-
ZTX451
ZTX551
Figure 7.21: H-bridge control circuit. Souce (G. O’Berto).
200
470
to PD5
To rotate the motor either clockwise or counter clockwise, a specific step sequence must be
sent to the motor control wires as shown in Figure 7.22c). As shown in Figure 7.22c) the control
sequence is transmitted by four pins on the microcontroller. In this example, we use PORTD[7:5].
The microcontroller does not have sufficient capability to drive the motor directly. Therefore,
an interface circuit is required as shown in Figure 7.22c). For a unipolar stepper motor, we employ
a TIP130 power Darlington transistor to drive each coil of the stepper motor. The speed of motor
rotation is determined by how fast the control sequence is completed. The TIP 30 must be powered
by a supply that has sufficient capability for the stepper motor coils.
Example: An ATmega324 has been connected to a JRP 42BYG016 unipolar, 1.8 degree per
step, 12 VDC at 160 mA stepper motor. The interface circuit is shown in Figure 7.23. PORTD
pins 7 to 4 are used to provide the step sequence. A one second delay is used between the steps
to control motor speed. Pushbutton switches are used on PORTB[1:0] to assert CW and CCW
stepper motion. An interface circuit consisting of four TIP130 transistors are used between the
microcontroller and the stepper motor to boost the voltage and current of the control signals. Code
to provide the step sequence is shown below.
Provided below is a basic function to rotate the stepper motor in the forward or reverse
direction.
//*************************************************************************
//target controller: ATMEL ATmega324
//
//ATMEL AVR ATmega324PV Controller Pin Assignments
//Chip Port Function I/O Source/Dest Asserted Notes