Servo Jumpy with DT Interrupt

[Dick Ivers]

Guys,

Thanks for the suggestions. I'll follow your tips and see where they lead. It the meantime, here is a sample of the code that causes the servo to twitch.

I've observed that the jumps occur about 2 times per second, and are in a direction that a longer pulse width would cause. This supports Jerson's idea that the interrupt service is periodically adding to the pulse width. Maybe it would help if the ISR were written is assembly and therefore be faster?

'****************************************************************
'*  Name    : servo_demo.pbp                                     *
'*  Author  : Dick Ivers                                         *
'*  Date    : Nov 30, 2010                                       *
'*  Notes   : This is a simplistic demo of servo w/Elapsed Timer *
'****************************************************************
'435 program words
'device is PIC 16F690
 '****************************************************************
'set device configuration
@ __config _HS_OSC & _WDT_ON & _MCLRE_ON & _CP_OFF

'set registers
        CM1CON0.7 = 0        'comparator1 off
        CM2CON0.7 = 0        'comparator2 off
        ANSEL = 0            'porta pins all digital 
        ANSELH = 0           'all porta pins digital

        DEFINE OSC 16          '16 Mhz oscillator
 
'declare variables
        led       var portc.5      'led output on portc.5
        servopos  var word
        runtime   var word         'motor runtime in tenths of seconds

start: 
        low led      'flash red LED for 0.5 sec to show circuit is functioning
        pause 500
        high led     'turn off led


INCLUDE "DT_INTS-14.bas"      'Base interrupt system
INCLUDE "ReEnterPBP.bas"      'Using PBP interrupts
INCLUDE "Elapsed_INT_RI2.bas" 'Elasped timer counting seconds and tenths seconds

ASM
INT_LIST  macro     ; IntSource,        Label,  Type, ResetFlag?
         INT_Handler   TMR1_INT,  _ClockCount,   PBP,  yes
    endm
    INT_CREATE            ; Creates the interrupt processor
ENDASM

@    INT_ENABLE  TMR1_INT         ; Enable Timer 1 Interrupts  

        runtime = 100             ' run servo for 10.0 seconds
        servopos = 600            ' hold at mid servo range (4*150)
        gosub ResetTime           ' Reset Time to  0d-00:00:00.00
        gosub StartTimer          ' Start the Elapsed Timer
        low Led                   'turn on led
        low portc.4               'turn on servo 
main:  
        pulsout portc.0,servopos
        pause 18
         if time => runtime then     'stop servo when target time is reached
         high led                    'turn off led
         goto hold
        endif
        goto main                   'loop until endtime is reached
        
hold:   
@    INT_DISABLE  TMR1_INT          ; Disable Timer 1 Interrupts  
        high portc.4                'turn off servo
        end