Make sure the wires are connected properly (black is ground, the [usually red] middle wire is positive, signal is often white or yellow)
Also, try using a separate power supply for the servo. It will draw quite a bit of current when it tries to move, possibly pulling the voltage down enough that the PIC resets or does something else unpleasant.

-------Original Message-------
From: Joel Middleton <chopsuey1973@YAHOO.COM>
Sent: 06/15/03 01:12 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: Servo Pulse Problem

>
> I found this schematic on the web and this is my exact
circuit except the servo signal is connected to pin 3
on portb. Also I have all the unused pins connected to
ground. Anyone see anything wrong with the circuit?


--- rneel@COX.NET wrote:
> I'm certainly no expert, but it looks like a classic
> capacitor discharge issue to me.  If you have a cap
> on this output, it will take time for it to
> discharge causing the slow decay in voltage instead
> of a nice sharp corner.  I can't see your circuit
> but do you have a cap connected across the pin you
> measured with the scope?
> >
> > From: Joel Middleton <chopsuey1973@YAHOO.COM>
> > Date: 2003/06/15 Sun AM 01:21:50 EDT
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: [PIC]: Servo Pulse Problem
> >
> > I believe I am having a problem with the pulse I
> am
> > sending to a servo connected directly to portb pin
> 3.
> > I have tmr0 set to overflow ever .0001 seconds.
> The
> > variable 'count' is incremented each time tmr0
> > overflows when count reaches 200 it is reset to 0.
> > When count is equal to zero I set portb pin 3 and
> when
> > count equals pw1 I clear the pin. I believe this
> > should give me a 1ms pulse every 20ms. This was
> not
> > working like I thought and I wanted to take a look
> at
> > the pulse so I downloaded one of those
> ocilloscopes
> > that runs through a sound card as I did not have
> > access to a real one. Well I found out that I am
> not
> > getting a nice square wave. The rising edge looks
> fine
> > but the falling edge is a slope. Can anyone tell
> me
> > how to correct this? I have included my code and
> also
> > attached an image I captured from the occiloscope.
> > Thanks.
> >
> >
> > LIST R=DEC
> >
> > INCLUDE "p16f84a.inc"
> >
> > CBLOCK 0x0C
> > _w
> > _status
> > _pclath
> > _fsr
> > count
> > pw1
> > ENDC
> >
> > __CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON
> >
> > push    macro
> >         movwf   _w                      ;Save
> registers
> >         movf    STATUS, w
> >         movwf   _status
> >         movf    PCLATH, w
> >         movwf   _pclath
> >         clrf    PCLATH
> >         endm
> >
> > pull    macro
> >         swapf   _pclath, w
> >         movwf   PCLATH
> >         movf    _status, w      ;Restore registers
> >         movwf   STATUS
> >         swapf   _w, f
> >         swapf   _w, w
> >         endm
> >
> > PAGE
> >
> >         org             0
> >         nop
> >
> >         goto    Main
> >
> >         org             4
> ;Interrupt
> >
> >         push
> >
> > Timer0
> >         btfss   INTCON, T0IF
> >                 goto    Iend
> >         movlw   165
> >         movwf   TMR0
> >         bcf             INTCON, T0IF
> >         incf    count
> >         movlw   200
> >         subwf   count, W
> >         btfss   STATUS, Z
> >                 goto    Step1
> >         movlw   0
> >         movwf   count
> > Step1
> >         movlw   0
> >         subwf   count, 0
> >         btfsc   STATUS, Z
> >                 bsf             PORTB, 3
> >         movf    pw1, W
> >         subwf   count, 0
> >         btfsc   STATUS, Z
> >                 bcf             PORTB, 3
> > Iend
> >
> >         pull
> >
> >         retfie
> >
> > Main
> >
> >         bsf             INTCON, GIE
> >         bsf             INTCON, T0IE
> >
> >         bsf     STATUS, RP0
> >         bcf             OPTION_REG, T0CS
> >         bsf             OPTION_REG, PSA
> >     bcf         STATUS,RP0          ; RAM PAGE 0
> >
> >         movlw   156
> >         movwf   TMR0
> >
> >         clrf    PORTB
> >
> >         bsf     STATUS, RP0
> >         bcf     TRISA ^ 0x080, 1
> >         bcf     TRISA ^ 0x080, 2
> >         clrf    TRISB
> >         bcf     STATUS, RP0
> >
> >         clrf    PORTA
> >         clrf    PORTB
> >         clrf    count
> >
> >         movlw   10
> >         movwf   pw1
> >
> > Loop
> >         nop
> > goto    Loop
> > end
> >
> >
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>

> ATTACHMENT part 2 image/pjpeg name=pulse.jpg



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