Decoding an incoming infrared signal: need advice

[xnihilo]

Here is the stuff:

I'm using PIC16F684 (as usual) and I connected a Vishay TSOP4840 IR receiver-demodulator.
It has 3 leads: GND, VSS and DATA.
The data lead connects to RA0 of my pic. RA0 is set as input and has its WPU enabled so when the receiver gets 40KHz IR signal at 940 nm (and amplified it) it brings RA0 low.

Well.

Now I have to write a routine that will decode the signal.

Signal is a PWM signal (40KHz, 30% duty cycle).
The data encoding is:

Header: High 2400us.

Data: 8 bits, sent from MSB to LSB, decoded from MSB to LSB.
Each byte is prefixed with a 600us no-pulse pause (the 'spacer'). If a bit is set, there is a 1200us pulse, if bit is cleared, there is a 600us pulse.

The routine I would like to use is:
-----------------------------------------------

WHEN INT OCCURS:

(inside my int handler, once i've cleared port latch mismatch anr reset int flag)

databyte = %00000000

'wait for the header pulse to end and check if duration is about 2400us+/-100us
j = 0
WHILE (porta.0 == 0)
  PAUSEUS 20
  j = j + 1
WEND
IF (( j < 115) || (j > 125)) then abort    'wrong pulse duration

FOR i = 0 to 7

j = 0  
WHILE (porta.0 == 1)    'wait until the bit prefix (600us no-pulse) ends
  PAUSEUS 20
  j = j + 1
WEND
IF ((j < 25) || (j > 35)) then abort    'wrong no-pulse duration, we want it to be 600us+/-100us

j = 0
  WHILE (porta.0 == 0)
    PAUSEUS 20
    j = j + 1
  WEND
  IF ((j > 25) && (j < 35)) THEN
    databyte.0(i) = 0
    GOTO bypass
  ENDIF
  IF ((j > 55) && (j < 65)) THEN
    databyte.0(i) = 1
    GOTO bypass
  ENDIF 
GOTO abort    'wrong pulse duration
bypass:

NEXT i

abort:

------------------------


I'm afraid this code is pretty heavy and I don't like the lack of accuracy in the timing that force me to use stupid code like:
IF ((j < 25) || (j > 35)) then abort

Is there a faster and more accurate way to decode my signal?

THANKS A LOT for any help or advice.

regards

charles

[mister_e]

Maybe you can find some tips with the following
http://rentron.com/PicBasic/IR_Chips.htm

[Bruce]

From the bit timing you indicate, I would guess this is a Sony IR transmitter? PULSIN should
be all you need to decode this type. It's one of the easiest around to encode & decode.

[xnihilo]

No guys,

I can't change my hardware.

BRUCE:
"From the bit timing you indicate, I would guess this is a Sony IR transmitter? PULSIN should
be all you need to decode this type. It's one of the easiest around to encode & decode."

It is a VISHAY TSOP4840 receiver (see mouser for the datasheet) and the emiter is a VISHAY TSAL6100 led. I'm using pic's PWM output to encode and I need to decode on the other side but do it quickly and acurately. I'm not using pulsin, I'm polling the input pin, what's wrong with that?

[Bruce]

Nothing's wrong with polling, if it works for you. I just prefer PULSIN since it's more
than accurate enough, and a lot easier.

[xnihilo]

Nothing's wrong with polling, if it works for you. I just prefer PULSIN since it's more
than accurate enough, and a lot easier.

I agree.

Thank you very much.