How to program pwm an arbitrary waveform to generate infrared stream

Question

I've use a pic to create IR patterns to modulate frequencies (37kHz or 58kHz) to control different devices. I'd like to move as much as possible to a RPi. I will use a pic to generate the 37kHz or 58kHz, but would ike to be able to have the RPi control the 'on' and 'off' periods of the pic signals.

I would like to create a waveform where, a logical '1' could be descibed as 2-600us periods followed by 1 600us low period. A logical '0' would be described as a 600 us high, followed by a 600us low.

If this is too complex, I can always drop back and use a pic to receive a 'command string' via RS232 from the RPi to do the waveform generation.

Thanks

Search on Raspberry Pi and [tag:PWM], the GPIO pins can also generate PWM. Maybe that way you can put all functionality into the RPi. — ikku, Dec 28 '12 at 16:21
On second thought, I fear that the RPi with Linux on it cannot handle the time constraints to manipulate the waveform. Interesting! — ikku, Dec 28 '12 at 16:36

score 2 · Answer 1 · edited Apr 09 '13 at 01:46

I have managed to do this with the Python RPIO library. I set the PWM up with a 1 uSec pulse, and loaded the entire IR code into memory, doing the 36khz modulation in software. Since this is using DMA, the waveform is pretty good. The one issue I am having is that it does take about 4 seconds to get 2 seconds of DMA buffer loaded into memory.

So, it is possible without too many issues. The code is not pretty, but does work :-)

  out += [header[0]]
  out += [header[1]]

      for c in send:
        if c == "1":
      out += [one[0]]
      out += [one[1]]
    else:
      out +=  [zero[0]]
      out +=  [zero[1]]
  out +=  [ptail]
  #out +=  (chr(0xff) + chr(0xff))

  # Pulses => Out
  #print out

  out2 = [0]
  running = 0

  for number in out:
    running += number
    out2 += [running]

  # Running Pulses => Out
  print out2

  #for i in range(len(out2)/2):
  #  a = out2[i*2]
  #  b = out2[i*2+1]

  pw = 1000000 / (36000 * 2) 

  print "PW %d" % (pw)
  print time.time()

  # Setup PWM and DMA channel 0
  PWM.setup(pulse_incr_us=1)
  PWM.init_channel(0, subcycle_time_us=2000000)



  high = True
  for a,b in pairwise(out2):
    #print "%d %d %d" % (high, a, b)


    if high:
      #pulse = int (a / pw)
      #PWM.add_channel_pulse(0, 17, a, pw)


      pointer = a
      while True:
        pStart = pointer
        pStop = pointer + pw
        if pStop > b:
          pStop = b
          #print "End of Packet 1"
        PWM.add_channel_pulse(0, 17, pStart, pStop-pStart)
        pointer += pw * 2
        if pointer > b: # was pStart > b; One too many cycles
          #print "End of Packet 2"
          break
    high = not high
  print time.time()

  time.sleep (30)

  # Stop PWM for specific GPIO on channel 0
  PWM.clear_channel_gpio(0, 17)

  # Shutdown all PWM and DMA activity
  PWM.cleanup()

score 1 · Answer 2 · answered Feb 12 '13 at 11:05

1

The PWM output is not really suitable to create arbitrary spaced high-low patterns. But have a look at the PCM audio output that can indeed do this and the frequency of 37kHz is in the range of the output capabilities.

answered Feb 12 '13 at 11:05

user1217949

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How to program pwm an arbitrary waveform to generate infrared stream

2 Answers2