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One basic application of PIC microcontrollers is their use to control motion based on input from a sensor. This is applicable to many different fields, from manufacturing to aeronautics to robotics. This tutorial will demonstrate the control of a Futaba servo motor using a PIC 16F84 microcontroller and input from a Devantech SRF04 ultrasonic sensor.
To do this, it is assumed that you already:
The rest of the tutorial is presented as follows:
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). The only additional parts you will require are:
| PART DESCRIPTION | VENDOR | PART | PRICE (2003) | QTY | |
| SRF04 Ultrasonic Sensor | Acroname | R93-SRF04 | 33.00 | 1 | |
| Futaba Servo Motor | RC Hobby Center | FUTM0031 | 21.99 | 1 | |
This sensor was chosen because of its compactness and the wide range over which it can measure. It is also easily interfaceable with microcontrollers. The servo chosen is a standard servo, however, any servo that operates off of PWM input will do (timing may vary).
To construct the circuit, you will also need:
The items listed above can all be purchased from an electronics store such as Radio Shack. Some hardware such as Home Depot carry tools like wire strippers and multimeters.
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Port A0 (Fig 1 - Pin 17) <=WIRED TO=> Output from sensor (Fig 2 - Output) Port B3 (Fig 1 - Pin 9) <=WIRED TO=> Input to sensor (Fig 2 - Input) Port B0 (Fig 1 - Pin 6) <=WIRED TO=> Command to servo (white wire)
This circuit will allow us to receive input from port A of the PIC and send output to port B. The ports were chosen to seperate inputs and outputs and to facilitate the insertion of other sensors. Different ports could be used, however, the code must be changed accordingly.
Figure 3 above shows the command signal that must be generated to begin a reading and the resulting output from the sensor. To initialize a reading, the command to the sensor must be held low, and then brought high for a minimum of 10 microsec. The pulse is generated on the falling edge of the command signal. After the command is given, the microcontroller must wait until it sees the output from the sensor go high. As soon as the output goes high, the microcontroller begins recording the lenth of the output signal until it sees the output go low again.
Control of the servo is achieved by generating a PWM signal. A PWM signal is simply a pulse of varying length that can be translated into a position requested of the servo. This is illustrated in Figure 4. Generally, the length of the pulse for a servo varies between 1 msec and 2 msec over a 20 msec period.
The following code requests a reading from the sensor, receives the reading, and transforms the reading into a signal that is outputted to the servo.
; FILE: sncserv.asm ; AUTH: Keith Sevcik ; DATE: 1/24/03 ; DESC: ; NOTE: Tested on PIC16F84-04/P ; ;---------------------------------------------------------------------- ; cpu equates (memory map) list p=16f84 radix hex ;---------------------------------------------------------------------- status equ 0x03 ; status equate porta equ 0x05 ; port a equate portb equ 0x06 ; port b equate PWM equ 0x0c ; PWM signal length count equ 0x0d ; general register temp equ 0x0e ; general register loop equ 0x0f ; general register ;--------------------------------------------------------------------- c equ 0 ; status bit to check after subtraction ;--------------------------------------------------------------------- ; porta0 = input from sensor ; portb3 = command to sensor ; portb0 = command to servo org 0x000 start movlw 0x00 ; load W with 0x00 make port B output tris portb ; port B is outputs movlw 0xFF ; load W with 0xFF make port A input tris porta ; port A is inputs movlw 0x00 ; load W with 0x00 to set intial value of B movwf portb ; set port b outputs to low main clrf count ; clear count clrf PWM ; clear PWM bsf portb,3 ; tell sensor to make reading movlw d'4' ; 4*3=12 clock delay movwf count call delay ; delay bcf portb,3 ; end sensor command signal movlw d'5' ; set the delay for measuring the output movwf count A0LOW btfss porta,0 ; if the output has gone high, skip the next instruction goto A0LOW ; else check again A0HIGH incf PWM ; increment the length of the PWM signal call delay ; delay for the count assigned above nop btfsc porta,0 ; check to see if the output is still high goto A0HIGH ; if it is, repeat movf PWM,w ; move PWM to w sublw d'200' ; subtract PWM cycle from 200 (2 msec) btfsc status,c ; if PWM is greater than 200 (2 msec), skip next instruction goto skip1 movlw d'200' ; else set the max PWM length to 200 movwf PWM skip1 movf PWM,w ; move PWM to w sublw d'20' ; subtract PWM cycle from 200 (2 msec) btfss status,c ; if PWM is greater than 200 (2 msec), skip next instruction goto skip2 movlw d'20' ; else set the min PWM length to 20 movwf PWM skip2 movlw d'1' ; set the delay for generating the PWM movwf count bsf portb,0 ; start the PWM pulse LoopPWM call delay nop nop nop decfsz PWM ; decrement the PWM length goto LoopPWM ; as long as PWM is greater than 0, loop bcf portb,0 ; when done looping, stop the pulse movlw d'15' ; set the counter for generating the rest of the PWM signal movwf loop del15 movlw d'255' ; set the delay counter movwf count call delay decfsz loop goto del15 goto main ;---------------------------------------------------------------------- delay movf count,w ; delay loop movwf temp del decfsz temp ; 3 clock cycles per delay loop goto del return ;---------------------------------------------------------------------- end ;---------------------------------------------------------------------- ; at burn time, select: ; memory uprotected ; watchdog timer disabled ; standard crystal (4 MHz) ; power-up timer onHEADER 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 ;---------------------------------------------------------------------- status equ 0x03 ; status equate porta equ 0x05 ; port a equate portb equ 0x06 ; port b equate PWM equ 0x0c ; PWM signal length count equ 0x0d ; general register temp equ 0x0e ; general register loop equ 0x0f ; general register ;--------------------------------------------------------------------- c equ 0 ; status bit to check after subtraction ;--------------------------------------------------------------------- ; porta0 = input from sensor ; portb3 = command to sensor ; portb0 = command to servo 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 ; port B is outputs movlw 0xFF ; load W with 0xFF make port A input tris porta ; port A is inputs movlw 0x00 ; load W with 0x00 to set intial value of B movwf portb ; set port b outputs to low
These lines set up port A as inputs and port B as outputs. All outputs are then set to low.
main clrf count ; clear count clrf PWM ; clear PWM bsf portb,3 ; tell sensor to make reading movlw d'4' ; 4*3=12 clock delay movwf count call delay ; delay bcf portb,3 ; end sensor command signal
After setting up the ports, the main loop is begun. At the beginning of the main loop, the count and PWM registers are cleared. The command pulse is then sent to the sensor.
movlw d'5' ; set the delay for measuring the output movwf count A0LOW btfss porta,0 ; if the output has gone high, skip the next instruction goto A0LOW ; else check again
The next bit of code sets up the counter for the next operation. This counter will add a delay to the process of measuring the output from the sensor. This delay will scale down the output from the sensor. The following two lines of code detect when the output from the sensor goes high.
A0HIGH incf PWM ; increment the length of the PWM signal call delay ; delay for the count assigned above nop btfsc porta,0 ; check to see if the output is still high goto A0HIGH ; if it is, repeat
This loop increments the PWM register, delays, and then loops again as long as the output from the sensor is high.
movf PWM,w ; move PWM to w sublw d'200' ; subtract PWM cycle from 200 (2 msec) btfsc status,c ; if PWM is greater than 200 (2 msec), skip next instruction goto skip1 movlw d'200' ; else set the max PWM length to 200 movwf PWM skip1 movf PWM,w ; move PWM to w sublw d'20' ; subtract PWM cycle from 200 (2 msec) btfss status,c ; if PWM is greater than 200 (2 msec), skip next instruction goto skip2 movlw d'20' ; else set the min PWM length to 20 movwf PWM
These lines set a max and min value for the PWM signal to prevent it from damaging the servo. It subtracts a value of 200 and 20 from the PWM signal and tests to see if there wasnt or was a carry, respectively. If the PWM length fails either test, it is set to either the max or min and the program continues.
skip2 movlw d'1' ; set the delay for generating the PWM movwf count bsf portb,0 ; start the PWM pulse LoopPWM call delay nop nop nop decfsz PWM ; decrement the PWM length goto LoopPWM ; as long as PWM is greater than 0, loop bcf portb,0 ; when done looping, stop the pulse
This code actually generates the PWM pulse. A delay length is stored in the count register. The output to the sensor is then set high. This brins the program into a loop that decrements the PWM register, delays, and then continues to loop so long as the value of the PWM register is greater than 0. After completing the loop, the output to the servo is brought low again.
movlw d'15' ; set the counter for generating the rest of the PWM signal movwf loop del15 movlw d'255' ; set the delay counter movwf count call delay decfsz loop goto del15 goto main
This final bit of code generates the remainder of the PWM signal. It consists of a delay nested inside a loop to complete the 20 msec period. When the loop has finished, the entire program is repeated.
After completing this tutorial you should be familiar with the SRF04 ultrasonic sensor, PWM control of a servo and be able to write code for a PIC 16F84 to control a servo based on input from an ultrasonic sensor.
If you have questions about this tutorial you can email me at Keithicus@drexel.edu.