The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Control Circuits
Listing 11-4. Controls direction and speed of a DC motor with BASIC-52’s
PWM output.
10 PORT1=0FBH
20 T=1
30 INPUT “Direction (0 or 1)? ”,D
40 INPUT “High (on) pulse width (25 to 0FFFFh)? ”,H
50 INPUT “Low (off time) pulse width (25 to 0FFFFh)? ”,L
60 IF D=0 THEN PORT1=PORT1.AND.0FEH ELSE PORT1=PORT1.OR.1
70 PRINT “Press any key to end program”
80 DO
90 PWM H,L,T
100 G=GET
110 UNTIL G0
120 PORT1=PORT1.AND.0FBH
130 END
You can stop the motor in any of three ways. You can bring bits 0-3 of the port low to stop
the timer. You can bring bit 7 high, which removes power from U4’s outputs. Or, you can
bring pins 9 and 10 of U5 high, which continues to apply power to the motor, but ignores
the step input.
Listing 11-3 prompts you for a motor speed, mode of operation, and direction, and then runs
the motor as requested. You can stop the program, and the motor will continue to run, as
long as it remains connected to the port and you don’t write anything else to it.
Speed Control of a Continuous DC Motor
If you prefer ordinary continuous dc motors to steppers, Figure 11-5 shows a circuit that
uses BASIC-52’s PWM output to control motor speed.
This circuit uses another Allegro chip, the UDN2993B (U1). Pins 1 and 8 of U1 connect to
the motor’s power supply, which can range from 10 to 40V. Pins 6 and 7 connect to the
motor. Pin 5 is the +5V logic supply, and pins 4, 5, 12, and 13 are additional grounds. At
pins 10, 11, 14, and 15, you can connect and control a second motor.
Pin 3 of U1 controls motor direction, and connects to bit 0 of Port 1 (P1.0) on the
8052-BASIC. Pin 2 of U1 switches power to the motor, with a logic low shutting the motor
off, and a logic high allowing current to flow. This pin connects to Port1, bit 2 (P1.2) of the
8052-BASIC, which is the PWM output.
BASIC-52’s PWM expression causes a series of pulses to appear at P1.2, with this syntax:
The Microcontroller Idea Book 195