Giving out pulses using propeller chip

prabhat

Hi,
I am trying to program a robot with servos on its joints which are controlled by a propeller. Its a HS 422 servo. It has a neutral at 1.5ms pulse. I am trying to give this pulse using the clkfreq and waitcnt but never getting it right. The servo who run to an extreme and hit a mechanical limit. I am not really sure if the pulses i am giving are of the right duration. 1.5ms every 20ms. the following is the code I used. Please help

CON
_clkmode = xtal1 + pll16x ' Feedback and PLL multiplier
_xinfreq = 5_000_000
Led = 1

PUB LedOnOff

dira[noparse][[/noparse]Led] := 1

repeat
outa[noparse][[/noparse]Led] := 1
waitcnt(clkfreq/667 + cnt)
outa[noparse][[/noparse]Led] := 0
waitcnt(clkfreq/50 + cnt)

Is there a problem with the code? I tried the servo with Basic stamp and that works fine.
Thank you in advance
Prabhat

JonnyMac

I wrote the attached program a couple months ago. It actually launches another Spin cog that serves as an 8-channel servo driver. As the whole thing is coded in Spin you might find it helpful.

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Jon McPhalen
Hollywood, CA

Post Edited (JonnyMac) : 8/4/2010 8:14:29 PM GMT

Javalin

have you looked at the servo objects in the OBEX? http://obex.parallax.com

It does look like your code is wrong.· Maybe :

us := clkfreq / 1_000_000   ' micro-seconds
ms := clkfreq / 1_000       ' milliseconds
 
twenty_ms := ms * 20        ' so we don't have to count each time
one_pt_five_ms := us * 1500
 
repeat
    outa[noparse][[/noparse]Led] := 1
    waitcnt(one_pt_five_ms + cnt)
    outa[noparse][[/noparse]Led] := 0
    waitcnt(twenty_ms + cnt)

Might be a simple way to do it.· Note the indentation!

edit - or this is a better way, as it provides a syncronised 1.5ms pulse - i.e. it begins every 20ms, rather than every 21.5ms as in previous loop.· (look at page 220 in the propeller manual)

VAR
    long    syncDelay
    long    ms, us
 
PUB Start
 
    us := clkfreq / 1_000_000     ' micro-seconds
    ms := clkfreq / 1_000         ' milliseconds
 
    twenty_ms := ms * 20          ' so we don't have to count each time
    one_pt_five_ms := us * 1500   '
 
    dira[noparse][[/noparse]Led]~~                   ' set to output
    syncDelay := cnt              ' setup syncronised delays
 
    repeat
        outa[noparse][[/noparse]Led] := 1
        waitcnt(one_pt_five_ms + cnt)     ' wait for 1.5ms
        outa[noparse][[/noparse]Led] := 0
        waitcnt(syncDelay += twenty_ms)   ' wait for the remainer of the 20ms

James

Post Edited (Javalin) : 8/4/2010 8:25:03 PM GMT

prabhat

I tried both the codes.. But surprisingly the same thing keeps happening. its driving me nuts because I am sure the mistake is something very fundamental. My servo would also wind up at one extreme and in fact pushing to go past it.

Javalin

what crystal and hardware board are you using?

james

prabhat

I am using the prop stick usb platform. It has a 5 Mhz built in crystal

W9GFO

Are you sure that you have the servo hooked up correctly?

waitcnt((clkfreq/1_000_000) * 1500 + cnt) is easier to understand (in my opinion) and will work at any _clkmode or _xinfreq without modification.

Rich H

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The Simple Servo Tester, a kit from Gadget Gangster.

Javalin

>waitcnt((clkfreq/1_000_000) * 1500 + cnt)
but requires the delay to be calculated each and every time you run the line.
>will work at any _clkmode or _xinfreq without modification.
so will mine.

I'll run up the code on my dev board tonight & put the o'scope to work on it.

Cheers,

James

Javalin

Code from the second example posted works fine - full code·as run on my·prop-demo-board·is:

CON
    _clkmode                = xtal1 + pll16x
    _xinfreq                = 5_000_000
    LED                     = 5
VAR
    long    syncDelay
    long    ms, us
    long    twenty_ms, one_pt_five_ms
 
PUB Start
 
    us := clkfreq / 1_000_000     ' micro-seconds (so we don't calc each time)
    ms := clkfreq / 1_000         ' milliseconds
 
    twenty_ms := ms * 20          ' so we don't have to calculate each time
    one_pt_five_ms := us * 1500   '
 
    dira[noparse][[/noparse]Led]~~                   ' set to output
    syncDelay := cnt              ' setup syncronised delays
 
    repeat
        outa[noparse][[/noparse]Led] := 1
        waitcnt(one_pt_five_ms + cnt)     ' wait for 1.5ms
        outa[noparse][[/noparse]Led] := 0
        waitcnt(syncDelay += twenty_ms)   ' wait for the remainer of the 20ms

Attachments to this post show the 1.5ms pulse (1ms per division) and the 20ms gap between the pulses (5ms per div).

Cheers,

James