The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Control Circuits
Listing 11-5. Controls direction and speed of Figure 11-6’s DC motor.
10 REM A=base address of 82C55
20 A=0FC00H
30 REM configure 8255 for all outputs
40 XBY(A+3)=80H
50 INPUT “Direction (0 or 1)? ”,D
60 IF D=0 THEN XBY(A+3)=0EH ELSE XBY(A+3)=0FH
70 INPUT “Speed (0-15)? ”,S
80 XBY(A)=S
90 REM bring 555’s Reset (PC.6) high (off)
100 XBY(A+3)=0DH
110 XBY(A)=S
120 G=GET
130 DO : G=GET : UNTIL G0
140 REM Remove 555’s Reset
150 XBY(A+3)=0CH
160 END
low pulses will be 10,000 microseconds, or 10 milliseconds, wide. For accurate time
calculations, set XTAL to match your crystal’s frequency.
In Listing 11-4, the number of cycles is 1, so the PWM statement results in one low pulse
followed by one high pulse. Lines 100-110 then use a GET operator to check to see if the
user has pressed a key and if not, the PWM output repeats. Checking for a key press enables
you to stop the program, since BASIC-52 ignores CONTROL+C keypresses while a PWM
statement is executing.
The DO loop in the program results in a delay between each PWM statement. During the delay,
P1.2 is high, so the motor is powered. Because of the delay, with a 12-Megahertz crystal, if
L=1200h and H=25, P1.2 will have equal on and off times of about 5 milliseconds. Increasing
L will lower the motor’s speed, and decreasing L or increasing H will raise it.
Although you can control a second motor with the ’2993B, you get only one PWM output
on the 8052-BASIC, so it’s not feasible to control two motors independently in this way.
Figure 11-6 shows another way to control the speed of a DC motor. As in Figure 11-4, the
circuit uses a 4066B quad analog switch to select a timing resistance for a 555 timer. Varying
the timing resistance varies the duty cycle of the 555’s output, and thus the motor’s speed.
You can select timing resistances that result in the motor speeds you want. The values shown
will vary the duty cycle from about 90 to 20 percent.
The Microcontroller Idea Book 197