Servo control woes

[Eriswerks]

I'm trying to get down the basics of PIC servo control, and not having much luck. I'm using a 16F690 and trying to get some kind of meaningful movement out of my servo. However, the only thing my servo will do is move to one extreme of its range and sit there, perhaps scornfully.

The offending code is:

@ DEVICE MCLR_OFF,WDT_OFF,PROTECT_OFF
DEFINE OSC 8
servo var PORTC.5
i var byte

loop:

for i = 200 to 400
low servo
pulsout servo,i
pause 18
next i

goto loop


I thought that would get a wiggle out of it, but no. I'm using the internal oscillator there, so I think my timing is correct - the 16F690 is running at 8MHz.

I've tried many different timings, and I've RTFM and RTFDataSheet and RTF-half-the-contents-of-google. I've tried so many tweaks and variations that I'm worried I'll hit the PIC's write limit soon. I've also tried using different servos (from other manufacturers) and a different 16F690. It just moves in one direction to its extreme and sits, every time.

What is it I am missing here? Any help would be greatly appreciated.

[Acetronics2]

[QUOTE=Eriswerks]I'm trying to get down the basics of PIC servo control, and not having much luck. I'm using a 16F690 and trying to get some kind of meaningful movement out of my servo. However, the only thing my servo will do is move to one extreme of its range and sit there, perhaps scornfully.

The offending code is:

@ DEVICE MCLR_OFF,WDT_OFF,PROTECT_OFF
DEFINE OSC 8
servo var PORTC.5
i var byte

loop:

for i = 200 to 400
low servo
pulsout servo,i
pause 18
next i

goto loop


hi,Eris

@ 8 Mhz, pulsout resolution is 5µS ( 10µS *4 / F.Mhz)

so, correct for that.

for i = 200 to 400 means i should not be a byte ( i max = 255 !!! )

so, what's happening ??? i value goes from 200 to 255 then ... back to 0 , out of the for - next loop !!! program ends.

just change i for a WORD !!!

Alain

[Eriswerks]

Thanks for the reply. I've changed the variable's type from BYTE to WORD, but there must be something else wrong as well - my servo still goes to one extreme and sits. Even if I do away with the loop entirely and just send it one position to center it, I'm still getting this behavior. That suggests to me that it must be timing-related, but I've tried timing some LED pulses without doing a DEFINE OSC and it confirms the 8MHz I see in the datasheet.

Maybe if I buy the PIC some chocolates?

[Eriswerks]

Looking through the 16F690 datasheet for anything I might have missed, I found this little nugget:

The output of the 8 MHz HFINTOSC and 31 kHz
LFINTOSC connects to a postscaler and multiplexer
(see Figure 3-1). The Internal Oscillator Frequency
select bits, IRCF<2:0> (OSCCON<6:4>), select the
frequency output of the internal oscillators. One of eight
frequencies can be selected via software.


So, it seems like in order to make sure that the PIC is running at 8MHz, I should be telling it to use HFINTOSC and not LFINTOSC, and according to the diagram mentioned I should be setting the frequency select bits to 111.

How do I go about doing that in PicBasic? Is it actually necessary to do so, or does the "intrc_osc_noclkout" that's in the include file for this chip have me covered already? I've also found mentions of putting "OSCCON = $70" in the code to set 8MHz, but I can't seem to find any real documentation on it. Is this controlling the frequency select bits? If so, do I still need to specify HFINTOSC somewhere?

Sorry for the deluge of questions...

[milestag]

1. Why not take the whole 8MHz issue out of the equation and run it at 4MHz? Once you get it working at 4MHz...THEN change the oscillator speed.

2. I have some servo code at home. For a 12F629 I think. I'll dig it out and post it later.

3. From what I remember, sending a single pulse of 1.5mS won't center the servo. You have to KEEP sending pulses to get it to move. So if it's at the extreme end, then you have to keep sending 1.5mS pulses until the servo reaches center. AND you need to keep sending those pulses if you expect it to STAY in the center. Some servos are quite a bit slower than others (check the spec sheet).

regards

[Dave]

Eriswerks, milestag is right, the servo requires updates every 50 milliseconds or so othwise most servos will shutdown and produce no torque.

Dave Purola,
N8NTA