Subject:
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Re: IR tower question
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 1 Dec 1999 18:43:13 GMT
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Viewed:
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589 times
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IrDA turns 'on' the IR for every 0 bit and holds it upfor ~ 1/2 of the
period of the transmission frequency.(more on http://www.irda.org)
BTW: For these reasons its better to set the IrDA transmission frequency to
2x38kHz=76kHz and send 0x55 or other similar bitpatterns. This will generate
a more acurate IR pattern in terms of timing of the single pulses on the
receiving side. Your milage may vary though...
On the IR tower side I understand that the (2400 baud) Tx line is basicly
just 'NAND'ed to the the 38kHz LED driving line.
Sending the bit pattern '10' generates the following IR pattern (ignoring
start and stop bits):
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_- 38Khz carrier
________________---------------- Bit pattern
________________-_-_-_-_-_-_- resulting IR pattern
Reading this via IrDA and @ 38kHz yields (again ignoring the important
start/stop bits):
0x00 0x00 0xff 0xff
Every 'IR tower' bit send will therefore generate 16 (38khz/2400) Ir peaks,
which needs to be read via IrDA.
Lets assume we want to read this via IrDA and are using a 38kHz IrDA
baudrate:
16 IR peaks @ 38KhZ have to be recognized and since IrDA requires a Start
Bit only 14 data bits can actually be read. You then have to configure your
IrDA receiver to use:
1 Start Bits
1 Stop Bits
6 Data Bits
Effectivly you use the first period of the 38kHz carrier as a (required)
start bit on the IrDA side!
Therefore you will effectivly receive two 6-bit values for every bit send by
the IR Tower
If you want to use an IrDA freq. of 76 khz the start/stop/data bit
distribution needs to be adapted...
HOWEVER, Since the Tower sends NOTHING out during the transmission of a 1
bit, you will have to use some timer device on the IrDA side to determine
this! Remember IrDA requires Start Bits.
The last point is IMHO really the killer for a Palm solution.
It should be actually easier to send IR data from the Palm to the RCX: Use a
76kHz transmission freq. and send 0x55 0x55 for every 0 Bit and 0xff 0xff
for every 1 bit. Do not forget to include the start/stop bits for the RCX
though! Since we use a 76Khz IrDA frequency, the sent out IrDA start Bits
should be too brief to be recognized by the RCX, whereas the 2bit patterns
in 0x55 should be recognized as one 38kHz carrier period.
Corrections welcome ..
Cheers
Reimer Mellin
Mike Kory <kory@advancenet.net> wrote in message
news:FLEFJ2.6z5@lugnet.com...
> Robert,
> I hope an electronics person tells me I'm wrong, because this is where I'm
> stuck too. The serial port sets 2400 baud. But my understanding is that we
> have to get the Palm to generate a 38kHz signal. That is, pulse the IR led
> on/off at 38kHz.
> So, someone tell me if I have this right: to send a '0' bit the Palm IR
> needs to pulse on and off at 38kHz for 1/2400 of second.
> Does the standard for IrDA just turn the IR on and off, or does it have a
> carrier frequency too?
> Mike
>
> Robert Eddings <webmake@bellsouth.net> wrote in message
> news:3833C702.F3E23CF0@bellsouth.net...
> > I realize that the signal sent is 38 kHz at 2400 baud but isn't that set
> > by the serial port?
> >
> > Robert Eddings
>
>
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Message has 1 Reply:
 | | Re: IR tower question
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| Thanks a lot, Reimer, for answering the Ir question. This gives a starting point for more playing around with the Ir. If only my palm had IR--maybe for Xmas. Really clever idea for creating the 38kHz carrier. Mike Reimer Mellin <davmel@primenet.com> (...) (25 years ago, 2-Dec-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: IR tower question
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| Robert, I hope an electronics person tells me I'm wrong, because this is where I'm stuck too. The serial port sets 2400 baud. But my understanding is that we have to get the Palm to generate a 38kHz signal. That is, pulse the IR led on/off at 38kHz. (...) (25 years ago, 18-Nov-99, to lugnet.robotics)
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