PWM Signal with PIC 16F84

[left][PWM Signal with PIC 16F84
Creating A PWM Signal Using A PIC 16F84
There are many small mechanisms, particularly servo motors, that use PWM coding as a
means of input. PWM signals can also be used to vary the voltage applied to a device by
achieving an effective average voltage. With so many applications, it is therefore necessary to
have a reliable means of generating a PWM signal.
MOTIVATION AND AUDIENCE
The focus of this tutorial is to demonstrate a method of generating a PWM signal using a PIC
16F84. This tutorial will teach you:
l What a PWM signal is.
l How to write code to generate a PWM signal using a PIC 16F84.
To do this, it is assumed that you already:
l Have completed "A Fast Track to PIC Programming".
The rest of the tutorial is presented as follows:
l Parts List and Sources
l Background
l Programming
l Applications
l Final Words
PARTS LIST AND SOURCES
In order to complete this tutorial you must have the circuit from the tutorial "A Fast Track to
PIC Programming" (minus the dip switches and resistor LED circuits). This circuit will be the
only part required for this tutorial. You will also need a DC power supply and access to an
oscilloscope to observe the signal.
BACKGROUND

PWM Signal with PIC 16F84
Figure 1
A PWM signal is simply a pulse of varying length, in effect a rectangular wave. This is illustrated
in Figure 1, which also shows how a servo might react to different PWM inputs. For our circuit,
the maximum voltage outputted will be +5 VDC, and the minimum will be 0 VDC. The length of
the pulse generated is some times charcterized by a duty cycle. The duty cycle is the
percentage of the signal that the output remains high. For instance, a constant +5V would be
equivalent to a 100% duty cycle. A typical square wave output from a function generator has a
50% duty cycle. 0V would correspond to a 0% duty cycle.
PROGRAMMING
PWM.asm
; FILE: PWM.asm
; AUTH: Keith Sevcik
; DATE: 5/21/03
; DESC: This program generates a PWM waveform.
; NOTE: Tested on PIC16F84-04/P
;----------------------------------------------------------------------
; cpu equates (memory map)
list p=16f84
radix hex
;----------------------------------------------------------------------

PWM Signal with PIC 16F84
portb equ 0x06 ; port b equate
duty equ 0x0c ; length of duty cycle
temp equ 0x0d ; length of duty cycle
;---------------------------------------------------------------------
c equ 0 ; status bit to check after subtraction
;---------------------------------------------------------------------
org 0x000
movlw 0x00 ; load W with 0x00 make port B output
tris portb ; copy W tristate to port B outputs
movlw 0x00 ; fill w with zeroes
movwf portb ; set port b outputs to low
rstrt movlw d'0'
movwf portb
movlw d'157' ; Duty cycle length
movwf duty
b0loop movf duty,w
movwf temp
bsf portb,0
pwma nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
decfsz temp
goto pwma
movlw d'255'
movwf temp
movf duty,w
subwf temp,f
bcf portb,0
pwmb nop

PWM Signal with PIC 16F84
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
decfsz temp
goto pwmb
goto rstrt
;----------------------------------------------------------------------
end
;----------------------------------------------------------------------
; at burn time, select:
; memory uprotected
; watchdog timer disabled
; standard crystal (4 MHz)
; power-up timer on
HEADER AND EQUATES
The first portion of code is the header and register equates. For more information about the
meaning of the header see the previous tutorial.
list p=16f84
radix hex
;----------------------------------------------------------------------
portb equ 0x06 ; port b equate
duty equ 0x0c ; length of duty cycle
temp equ 0x0d ; length of duty cycle
;---------------------------------------------------------------------
c equ 0 ; status bit to check after subtraction

PWM Signal with PIC 16F84
;---------------------------------------------------------------------
org 0x000
The only equate of signifficance here is PWM. This register will be used to store the length of
the PWM signal to be generated.
INSTRUCTIONS
The next portion of code contains the actual instructions that tell the PIC what to do.
start movlw 0x00 ; load W with 0x00 make port B output
tris portb ; copy W tristate to port B outputs
movlw 0x00 ; fill w with zeroes
movwf portb ; set port b outputs to low
These lines set up port B as outputs. All outputs are then set to low.
rstrt movlw d'0'
movwf portb
movlw d'157' ; Duty cycle length
movwf duty
After setting up the ports, the main loop is begun. At the beginning of the main loop, all port b
pins are set to low just incase they are high when they shouldn't be. The duty cycle is then set
to 157 (a 50% duty cycle. 255 corresponds to 100% and 0 corresponds to 0%).
b0loop movf duty,w
movwf temp
bsf portb,0
pwma nop
nop
nop
nop
nop
nop
nop
nop
nop
nop

PWM Signal with PIC 16F84
nop
decfsz temp
goto pwma
The next bit of code is the loop for the PWM signal generated at pin B0. The pwm1a loop
generates the high portion of the PWM signal. The duty cycle is stored in temp and then the pin
is set high. after a pause, temp is decremented and so long as it doesnt reach zero the pause is
repeated and temp is decremented again. After temp reaches zero, the code continues.
movlw d'255'
movwf temp
movf duty,w
subwf temp,f
bcf portb,0
pwmb nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
decfsz temp
goto pwmb
goto rstrt
The next portion of code generates the low part of the PWM signal. The value 255 is stored in
temp, and the duty cycle is subtracted from this. This gives the remaining length of signal to be
generated. Temp is then decremented in the same manner as above, this time with B0 set to
low. Once the entire PWM signal has been generated, the code repeats.
This code causes a PWM signal to be generated with a duty cycle proportional to the value set.
The frequency of the signal can also be adjusted by varying the delay (the number of nop's
used).
APPLICATIONS
One common application of pwm signals is motor control. By varying the duty cycle of a pwm
signal sent to a motor, you can vary the effective power of the signal and thereby slow the
http://www.pages.drexel.edu/~kws23/tutorials/PWM/PWM.html (6 of 8)14.03.2006 16:51:58
PWM Signal with PIC 16F84
motor down or speed the motor up depending on how long of a pulse you send to the motor.
The signal generated by the PIC can not be directly connected to the motor, however, because
the PIC is unable to handle the power required by the motor. It is therefore necessary to use a
transistor to regulate the flow of current to the motor. A transistor is like an electric switch.
When you send a logic high (+5V) to the transistor, it allows current to flow. When a logic low
(0V) is sent, it restricts the flow of current. For digital signals, this means that the signal can be
reproduced exactly, except the new signal is scaled up to a much larger current. Figure 2
shows a schematic for controlling a motor using a TIP31 NPN transistor.
Figure 2
As the schematic shows, the output from the pick is wired to the base. The negative terminal of
the motor is then connected to the base and the collector is connected to ground. When the
PWM otuput from the PIC is sent to the transistor, it will flip the transistor on and off and
subsequently generate the same PWM signal to the motor, allowing you to control the motor
with a PWM signal.