The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 11
PWM high pulse width, low pulse width, number of cycles
Listing 11-4 uses the PWM output to control motor speed. The program prompts you for
motor direction and the width of the high and low PWM pulses. For faster speeds, use large
values for the high pulses (H) and small values for the low pulses (L). This results in a
waveform with a high duty cycle, or ratio of the width of a high pulse to the width of a
complete cycle (consisting of 1 high and 1 low pulse). With a high duty cycle, power is
applied to the motor for a large proportion of the total time. For slower speeds, do the reverse:
select small values for the high pulses and large values for the low ones, for a low duty cycle.
The actual pulse width equals the value in the PWM statement multiplied by 12, divided by
the frequency of the 8052-BASIC’s timing crystal. So, with a 12-Megahertz crystal, if
H=1000 and L=10,000, high pulses will be 1000 microseconds, or 1 millisecond, wide, and
Figure 11-6. A 4066B quad switch and 555 timer enable you to select up to
16 motor speeds, with four port bits.
196 The Microcontroller Idea Book