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:

To do this, it is assumed that you already:

The rest of the tutorial is presented as follows:

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

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

;----------------------------------------------------------------------

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
	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

;---------------------------------------------------------------------

	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
	nop
	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 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.

FINAL WORDS

After completing this tutorial you should be familiar with PWM signals and how to program a PIC 16F84 to generate them.

If you have questions about this tutorial you can email me at Keithicus@drexel.edu.