[SOLVED] inverting the output pin connected to CTRA??

laser-vector

Hi,

I have a servo H-Bridge that i built acording to the schematic below. it seems to be a little backwards compared to most of the others ive seen where the Enable pin is an active low instead of High.



H-Bridge Truth Table:
_______________________________________________________
FWD REV ENA*
1 0 0 - Turn on upper left source and lower right sink. (go forward)
1 0 1 - Disable lower right sink. When fed a PWM signal the bridge modulates the "forward" current through the motor.
1 1 0 - Turn on both lower left sink and lower right sink, shorting the motor "Braking."
1 1 1 - Disable both lower sinks. When fed a PWM signal the bridge modulates the "braking" of the motor.
0 1 0 - Turn on the upper right source and lower left sink. (go backward)
0 1 1 - Disable lower left sink. When fed a PWM signal the bridge modulates the "reverse" current through the motor.
0 0 0 - Turn off all sources and sinks. "Coast" motor is not engaged at all.
0 0 1 - Turn off all sources and sinks in a different way, same effect though.
_______________________________________________________

ok, so when i use a method like this:

PUB PWM(pin) | x, n , deadline , onepercentticks     

dira[pin]~~
deadline := cnt
onepercentticks := clkfreq/30_000
repeat
  waitcnt(deadline += onepercentticks*100) 
  ctra := 0
  frqa := 1
  phsa := -(onepercentticks * ( 1 #> DUTY <# 100))
  ctra := (%0_00100 <<26) + pin

by setting DUTY to 100
the output would be constantly high (or 1) and the motors are Not Enabled
and by setting DUTY to 0
the output would be constantly low (or 0) and the motors are Enabled

so my H-bridge needs the complete opposite!

i was successful at writing a program where 100 = "stopped" and 0 = "full power"
but things were getting messy in a hurry.

the only way i can think of doing this is to use something like a 74LS04 Not Gate connected to the output pins, but i keep telling myself there's an easier way to accomplish this!
if anyone has any suggestions on how to invert the pin's output controlled by cntra's output i would greatly appreciate it!!

Thanks

Mike G

Use an inverter? 74HC04 if memory servers. <- From duplicate post

Don't the counter have a NOT mode where one is the inverse of the the other? I thought I remember reading that in the manual.

kuroneko

laser-vector wrote: »

if anyone has any suggestions on how to invert the pin's output or cntra's output i would greatly appreciate it!!

Use NCO differential (%00101) and use pin B instead of A? Changing the PWM logic should work as well.

Ariba

You can invert the PWM output by counting down and set a positive pulswith value:

PUB PWM(pin) | x, n , deadline , onepercentticks     

dira[pin]~~
deadline := cnt
onepercentticks := clkfreq/30_000
repeat
  waitcnt(deadline += onepercentticks*100) 
  ctra := 0
  [COLOR="#FF0000"]frqa := -1[/COLOR]
  [COLOR="#FF0000"]phsa := onepercentticks * ( 1 #> DUTY <# 100)[/COLOR]
  ctra := (%0_00100 <<26) + pin

..and you don't need to set the counter new in every loop, just once before the repeat loop:

PUB PWM(pin) | x, n , deadline , onepercentticks     

dira[pin]~~
deadline := cnt
onepercentticks := clkfreq/30_000
ctra := (%0_00100 <<26) + pin
[COLOR="#FF0000"]frqa := -1[/COLOR]
repeat
  waitcnt(deadline += onepercentticks*100) 
  [COLOR="#FF0000"]phsa := onepercentticks * ( 1 #> DUTY <# 100)[/COLOR]

Andy

laser-vector

Thanks for all the replies!

as soon as i get home today i will investigate these suggestions and post the results.

p.s.
I apologize for these novice questions, i am a beginner at using the counters but definitely see their usefulness and really want to learn! so thanks for baring with me

tonyp12

Are you using Counter Duty mode (Pulse-density modulation) to pwm the motor controller?
No way that will work, as it's to fast with 12.5ns pulses.
No h-bridge switch that fast and when going through opto-isolators will smooth out the signal even more.

Using NCO will give slower square waves, but fixed at 50% duty.

laser-vector

i managed to achieve this by configuring the BPIN and using NCO Differential as per the advice of kuroneko. but thank you all so much for your advice!!