Added subject tag...


>an LED graph.  Even though I'm sending a whole byte over serial, I'm only
>actually using the five least significant bits of the byte.

So how does the receiver know that when the first bit has been sent?

For example, the receiver won't see any action at all until the
transmitter tries to send a 1, right?  The receiver than takes that
bit as the first of eight, waits the precalculated time period, interprets
the next bit, repeat as needed.

What you'll actually get is a left shifted version of the sent byte, which
is shifted such that the first bit is the highest order 1 you wanted to
send, followed by the remaining bits of the number and wrapped up
with padded zeros.  For example, b'00011010' would be received as
b'11010000',  b'00000011' would be received as b'11000000', and
so on.

The trick here is that you need to use some kind of alternate signal,
bit-bashed out, or some very clever data manipulation with the UART.
Either non-return-to-zero encoding, or pulse length encoding, or any
of the other methods used by commercial products to communicate
via IR can be used.  Snoop on Google for IR remote control PIC and
you'll get some good ideas to start with.

The simplest answer is to make the first bit of your data always 1,
since you've indicated that that's a don't care bit anyway.  Thus,
the first bit will always trigger a reception on the receiver's USART,
and as long as the timing is good (which you have a strong indicator
of, since wired serial works), you should be in business.

Note that this is some speculation on my part, since I've never tried
this, but I do know for sure that it's not too tough to emulate the
Sony IR protocol, since I just made a receiver/transmitter pair that
does just that!  That could be your fall-back method, but it requires
more bit-bashing and what-not than using the USART.  Good luck!

Mike H.

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