Re: [Gumstix-users] Small 8ohm 0.1W speaker on PWM

[Arnar B. <ar...@gm...>]

Hi again,

On 3/13/07, Arnar Birgisson <ar...@gm...> wrote:
> I'd like to use one of the PWM outputs on the robostix to play a tone
> on a very small 8ohm 0.1W speaker I pulled from an old modem. I
> figured out the software side and setting up the timers, but I don't
> know how to safely wire up the speaker to the pins. Any pointers to
> some material on this?

I took a leap of faith and wired it through a 470ohm resistor (gnd ->
speaker -> resistor -> pwm signal pin). Should give me ~10mA. Seems
(or rather sounds) to work very nicely - I just played F=FCr Elise on it
with my midi keyboard plugged on /dev/ttyS3 :oD

In the odd chance that anyone's interested here's a little python
program that parses the midi and plays the note on the speaker:

import sys
import inspect
import serial
from i2c_io import I2C_IO_Device

midi_cmds =3D {}

def midihandler(status):
    def decorator(f):
        args =3D len(inspect.getargspec(f)[0])
        midi_cmds[status] =3D (f, args-1)
        return f
    return decorator


robo =3D None
notemap =3D None
noteplaying =3D None

@midihandler(0x80)
def note_off(channel, pitch, velocity):
    if pitch =3D=3D noteplaying:
        robo.write_reg('tccr1a', 0x00)
        robo.write_reg('portb', 0x00)

@midihandler(0x90)
def note_on(channel, pitch, velocity):
    global noteplaying
    if velocity =3D=3D 0:
        return note_off(channel, pitch, velocity)
    robo.write_reg('ocr1a',  notemap[pitch])
    robo.write_reg('tccr1a', 0x40)
    robo.write_reg('tccr1b', 0x0a)
    noteplaying =3D pitch

#  @midihandler(0xA0)
#  def aftertouch(channel, pitch, pressure):
#      print "Channel: %02d    Aftertouch   pitch=3D%d  pressure=3D%d" %
(channel, pitch, pressure)
#
#  @midihandler(0xB0)
#  def controller_change(channel, ctrlnum, value):
#      print "Channel: %02d    Ctrl change   ctrl=3D%d  val=3D%d" %
(channel, ctrlnum, value)

def isStatus(byte):
    return byte & 0x80

def parse_midi(stream):
    status, channel, cmd_info, data =3D None, None, None, []
    b =3D stream.next()
    while True:
        if isStatus(b):
            status =3D b & 0xf0
            channel =3D b & 0x0f
            data =3D []
            cmd_info =3D midi_cmds.get(status, None)
        elif status is not None:
            # it's a databyte
            data.append(b)

            # if we have enough data bytes, process the command
            if cmd_info and len(data) =3D=3D cmd_info[1]:
                cmd_info[0](channel, *data)
                data =3D []

        b =3D stream.next()

def eat_stream(s):
    while True:
        s.next()

def get_midi_freq(note):
    # see http://www.borg.com/~jglatt/tutr/notefreq.htm
    return (440.0 / 32) * (2 ** ((note - 9) / 12.0))

def get_ocr_value(freq):
    # 2000000 here is 16MHz divided by prescaler 8
    return 2000000 / (freq * 2)

def serialstream(s):
    while True:
        b =3D ord(s.read(1))
        yield b

if __name__ =3D=3D "__main__":

    notemap =3D [int(round(get_ocr_value(get_midi_freq(n)))) for n in range=
(127)]

    robo =3D I2C_IO_Device(0x0b)

    robo.write_reg('ddrb', 0xff)

    s =3D serial.Serial('/dev/ttyS2', 38400)
    parse_midi(serialstream(s))


Arnar