someone can give the algorithm to control a servo with PWM on pic16f84a please

 

or dear...
i don't think any one has any thing to give.
but we can discuss

 

You need to send a 1-2ms pulse @ 5V every 20ms.
The width of the pulse controls the position of the servo.

If you haven't already bought it, consider a more recent, better and probably cheaper PIC like the 16F628.

 

Just found this old file of mine that controls 4 servos with a pretty crazy method. It does work, but understanding how it does might be tricky.

    list    p=16F84a
    radix    hex
    title "SERVO"
        #include <p16f84a.inc>
    __config _CP_OFF & _PWRTE_OFF & _WDT_OFF & _HS_OSC

;    servo control 1-2ms pulse every 20ms 
    cblock 0x0C
    W_TEMP
    STATUS_TEMP

    servo0            ;pin rb0 position 0-255
    servo1
    servo2
    servo3            ;pin rb3
    intservogoto
    
    endc
    

;    org 0x2100        ;initial values for timer H and L bytes (10 seconds)
;    de 0x00,0x0a    ;first 2 bytes of eeprom

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

    org    0x000
    goto init
    org 0x004    ;interrupt goes here
    MOVWF W_TEMP ; Copy W to TEMP register,
    SWAPF STATUS, W ; Swap status to be saved into W
    MOVWF STATUS_TEMP ; Save status to STATUS_TEMP register
         ; Interrupt Service Routine
    
;writes intservergoto to program counter to send timing pulses to 4 servos in sequence
;without any messy logic. Initial value of intservergoto 09h. Timing is for 4MHz clock
;0 in servo variables gives about 2ms pulse, 254 gives about 1ms
;Don't use 255 for servo value 
;Advantages: fast, not many files used
;Disadvantages: uses lots of program memory, hard to modify

    movfw intservogoto    
    movwf PCL
    call prescaler4        ;PCL 09h        servo0
    bsf PORTB,0            ;start pulse and next interrupt after 1ms
    movlw 0E
    movwf intservogoto
    goto endint
    movfw servo0        ;PCL 0E
    movwf TMR0            ;variable part of pulse 0-1ms
    movlw 13
    movwf intservogoto
    goto endint
    call prescaler16    ;PCL 13
    movlw 30            ;for 3.5ms pause before next servo
    movwf TMR0
    bcf PORTB,0            ;end pulse
    movlw 1A
    movwf intservogoto
    goto endint
    call prescaler4        ;PCL 1A        servo1
    bsf PORTB,1            ;start pulse and next interrupt after 1ms
    movlw 1F
    movwf intservogoto
    goto endint
    movfw servo1        ;PCL 1F
    movwf TMR0            ;variable part of pulse 0-1ms
    movlw 24
    movwf intservogoto
    goto endint
    call prescaler16    ;PCL 24
    movlw 30            ;for 3.5ms pause before next servo
    movwf TMR0
    bcf PORTB,1            ;end pulse
    movlw 2B
    movwf intservogoto
    goto endint
    call prescaler4        ;PCL 2B        servo2
    bsf PORTB,2            ;start pulse and next interrupt after 1ms
    movlw 30
    movwf intservogoto
    goto endint
    movfw servo2        ;PCL 30
    movwf TMR0            ;variable part of pulse 0-1ms
    movlw 35
    movwf intservogoto
    goto endint
    call prescaler16    ;PCL 35
    movlw 30            ;for 3.5ms pause before next servo
    movwf TMR0
    bcf PORTB,2            ;end pulse
    movlw 3C
    movwf intservogoto
    goto endint
    call prescaler4        ;PCL 3C        servo3
    bsf PORTB,3            ;start pulse and next interrupt after 1ms
    movlw 41
    movwf intservogoto
    goto endint
    movfw servo3        ;PCL 41
    movwf TMR0            ;variable part of pulse 0-1ms
    movlw 46
    movwf intservogoto
    goto endint
    call prescaler16    ;PCL 46
    movlw 30            ;for 3.5ms pause before next servo
    movwf TMR0
    bcf PORTB,3            ;end pulse
    movlw 09
    movwf intservogoto


    goto endint

prescaler4
    bsf     STATUS, RP0    ; bank 1
    movlw   B'10000001'     ; prescaler 4
    movwf   OPTION_REG      ;  
    bcf     STATUS, RP0    ; bank 0
    return

prescaler16
    bsf     STATUS, RP0    ; bank 1
    movlw   B'10000011'     ; prescaler 16
    movwf   OPTION_REG      ;  
    bcf     STATUS, RP0    ; bank 0
    return

endint

     ; should configure Bank as required
     ;
    SWAPF STATUS_TEMP,W     ; Swap nibbles in STATUS_TEMP register
    ; and place result into W
    MOVWF STATUS             ; Move W into STATUS register
    ; (sets bank to original state)
    SWAPF W_TEMP, F         ; Swap nibbles in W_TEMP and place result in W_TEMP
    SWAPF W_TEMP, W         ; Swap nibbles in W_TEMP and place result into W
    clrf INTCON                ;clr interupts
    bsf    INTCON,T0IE            ;enable timer interupt
    retfie
    
init    ;initialise stuff here
    ;sets interupts only on timer0 internal and prescaler 256
    
    bsf     STATUS, RP0    ; bank 1
    movlw   B'10000111'     ; rtcc inc = tcylc/256 = tclk/(4*256)
    movwf   OPTION_REG      ;  
    bcf     STATUS, RP0    ; bank 0

    movlw 09
    movwf intservogoto

    clrf    TMR0            ; reset timer (and prescaler!)
    movlw   B'10100000'     ; enable timer interrupt and GIE
    movwf   INTCON          ;  
    
    BCF STATUS, RP0 ; bank 0
    CLRF PORTB                 ; Initialize PORTB by
                            ; clearing output
                            ; data latches
    BSF STATUS, RP0         ; Select Bank 1
    MOVLW B'11110000'         ; Value used to
                            ; initialize data
                            ; direction
    MOVWF TRISB             ; Set RB<0-3> as output, all others input
                            ;porta should all be inputs anyway
    BCF STATUS, RP0         ;bank 0


start
    
    goto start




    end