Does your data *has* to be 8-bit, 1 start, 1-2 stop just as
a normal RS232 communication ?

I implemented an IR comm using the following steps.

- Set up the PWM to generate the carrier (36 Khz in my case).

- Wrote a couple of subs to send "start", "1", "0" and "stop" bits.
  The subs just enable/disable the PWM generator. Each bit is made up
  from a "on" and "off" time with different periods for each type
  of bit. So the PWM carrier is further PWM modulated with the data,
  in a way. The ons and offs are synced with the tmr2 overflow to
  get "clean" transitions and stable on and off times.

- Created a "protocol" with about 12-14 bits of data (divided
  into "unit", "command", "value" and so on).

- The recieving PIC waits for a rising edge from the IR receiver (TSOP)
  and then times the "on" period using a timer. The timer is stopped
  when the "on" period finish.

- The "on" periods for the different bit types are such as
  each bit type corresponds to a specific bit in the timer beeing set.
  So it's just a number btfss (or btfsc) on the timer value to decode
  the different bits. Easy.

- There is also a check bit at the end as a simple error checking.

Each block is sent three times, and the receiver needs a correct
check bit to accept the block. There is also a "block number" in the
data to protect from multiple readings of the same block.

The sender is a 18F1220 and the receiver is a 12F629, both running
on the internal osc block.

Jan-Erik.

-
> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Ishaan Dalal
> Sent: den 4 december 2003 10:49
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC:] IR serial transmission
>
>
> As part of an (academic) project, I need to implement
> point-to-point IR
> transmission using an async serial format. As I understand it, IR
> transmissions must use a carrier for preventing ambient IR
> interference. 38
> KHz seems to be pretty common.
>
> I will be using a 16F877 [mandated :-( ]. The AUSART would be
> ideal for
> baseband stuff, and with an external 38 KHz clock and an AND
> gate, this
> would be simple. But we need to do as much in software as
> possible, so I was
> wondering if there was some clever way to directly let the
> USART control the
> IR LED, since the modulation is simply ASK.
>
> I thought the PWM module could be used to generate the 38 KHz
> carrier, but
> how would I use the USART to switch the duty cycle between 0 and 50%,
> without some sort of external feedback connection?
>
> Another approach might be to run the USART at ~38.4 KHz and
> "send" 0xAAs and
> 0x00s to emulate 38 KHz ASK....
>
> Of course, if I just used the PWM and wrote a bit-banged
> custom USART these
> questions wouldn't arise, but I would love to use the
> hardware USART.....
>
> FYI, the source data is from a PC parallel port.
>
> Cheers,
> Ishaan
>
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