Specifications

BASIC Stamp II

Parallax, Inc. • BASIC Stamp Programming Manual 1.8 • Page 293

PWM

pin, duty, cycles

Convert a digital value to analog output via pulse-width modulation.

•

is a variable/constant (0-15) that specifies the I/O pin to use.

This pin will be placed into output mode during pulse generation

then switched to input mode when the instruction finishes.

•

Duty

is a variable/constant (0-255) that specifies the analog

output level (0 to 5V).

•

Cycles

is a variable/constant (0-65535) specifying an approximate

number of milliseconds of PWM output.

Explanation

Pulse-width modulation (PWM) allows the BS2—a purely digital de-

vice—to generate an analog voltage. The basic idea is this: If you make

a pin output high, the voltage at that pin will be close to 5V. Output

low is close to 0V. What if you switched the pin rapidly between high

and low so that it was high half the time and low half the time? The

average voltage over time would be halfway between 0 and 5V—2.5V.

This is the idea of PWM; that you can produce an analog voltage by

outputting a stream of digital 1s and 0s in a particular proportion.

The proportion of 1s to 0s in PWM is called the duty cycle. The duty

cycle controls the analog voltage in a very direct way; the higher the

duty cycle the higher the voltage. In the case of the BS2, the duty cycle

can range from 0 to 255. Duty is literally the proportion of 1s to 0s

output by the PWM instruction. To determine the proportional PWM

output voltage, use this formula: (duty/255) * 5V. For example, if duty

is 100, (100/255) * 5V = 1.96V; PWM outputs a train of pulses whose

average voltage is 1.96V.

In order to convert PWM into an analog voltage we have to filter out

the pulses and store the average voltage. The resistor/capacitor com-

bination in figure I-12 will do the job. The capacitor will hold the volt-

age set by PWM even after the instruction has finished. How long it

will hold the voltage depends on how much current is drawn from it

by external circuitry, and the internal leakage of the capacitor. In order

to hold the voltage relatively steady, a program must periodically