Servo control with 12F629

I'm having some trouble with a continuous rotation servo and a 12F629. I've managed to have a BS2 control the servo at different speeds in both directions, but everytime I try to use the 12F629 to control the servo, it only operates in one direction (counter-clockwise) at the same speed (fast). A typical program I'd use with the BS2 was:

loop
pulsout 0, 750
pause 20
goto loop

The code I used for the 12F629 was:

loop
pulsout 0, 150
pause 20
goto loop

I tried varying the 150 to 100, 180, 750, etc... with the same direction and speed everytime. I'm using the 12F629's internal RC oscillator (4Mhz), which is why I use the 150 instead of the 750 (for the BS2). I even included a "Define OSC 4" to make sure the pulsout was correct.

Does anyone have any suggestions or advice as to what I'm doing wrong?

Thanks in advance!
Dave

P.S. Also, everytime I touch any of the leads with my finger (ground, +5V, etc), the servo stops. Is this because my finger is interfering with the capacitance of the internal RC oscillator, effectively stopping the 12f629?

Turn off analog comparators so your pins can be used for digital I/O.

CMCON = 7

PULSOUT toggles the pin twice, so the initial state of the pin determines
the logic of the pulse.

LOW 0 ' We want a high-going pulse or low-high-low with pulsout.

Code:

---

@ DEVICE MCLR_OFF,INTRC_OSC,WDT_OFF,PROTECT_OFF

Define        OSCCAL_1K 1  ' Load factory value into OSCCAL
CMCON = 7                  ' Turn analog comparators off

X VAR BYTE
LOW 0        ' Setup GPIO,0 to output a high-going pulse with pulsout

Main:
    FOR X = 1 to 50
    pulsout 0, 100
    pause 15
    NEXT X
   
    pause 1000
   
    FOR X = 1 to 50
    pulsout 0, 200
    pause 15
    NEXT X
   
    pause 1000
   
    goto Main

    end

---

You may want to tweak the pulsout periods, but that should get you started.

Make sure you use a separate power supply for your servo. Using the same PS as the PIC will cause problems.

Bruce,
Thanks for the help! The CMCON = 7 I think is what did the trick (i.e. it works). However, it works only intermittently. It seems that the microchip is only executing the full program when I have my finger on the +5V lead (and then only sometimes). I'm wondering what's causing this? I'm using the INTRC (IN) configuration option. I am also using the same PS for the servo and the chip (wall wart to a 1A, 5V regulator). Is the cause of this most likely due to sharing of the power source, a touchy internal oscillator, or something else?

Thanks again for the help.

Dave

Quote:

---

Originally Posted by Bruce

Turn off analog comparators so your pins can be used for digital I/O.

CMCON = 7

PULSOUT toggles the pin twice, so the initial state of the pin determines
the logic of the pulse.

LOW 0 ' We want a high-going pulse or low-high-low with pulsout.

Code:

---

@ DEVICE MCLR_OFF,INTRC_OSC,WDT_OFF,PROTECT_OFF

Define        OSCCAL_1K 1  ' Load factory value into OSCCAL
CMCON = 7                  ' Turn analog comparators off

X VAR BYTE
LOW 0        ' Setup GPIO,0 to output a high-going pulse with pulsout

Main:
    FOR X = 1 to 50
    pulsout 0, 100
    pause 15
    NEXT X
   
    pause 1000
   
    FOR X = 1 to 50
    pulsout 0, 200
    pause 15
    NEXT X
   
    pause 1000
   
    goto Main

    end

---

You may want to tweak the pulsout periods, but that should get you started.

Make sure you use a separate power supply for your servo. Using the same PS as the PIC will cause problems.

---

You don't want to run a servo from the same power supply your PIC runs on unless it's a bench type power supply or one that can handle a ton of current without dropping its output.

Use a separate supply or batteries for the servo motor. Should take care of the problem.

You might also want to strap a 0.1uF cap across the PIC Vcc & gnd pins too. Helps filter spikes from the supply, and it can keep the PIC from resetting at odd times.