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<H1 ALIGN="CENTER">A deeper look inside common Consumer
Infrared (CIR)</H1>
<center>by Brett Johnson</center>
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<p>
A common CIR receiver chip includes more than a
simple infrared photo-transistor (usually at
950nm). It includes demodulation circuitry
around a specific carrier frequency (typically
38kHz) and a latch circuit for its data output
pin that is triggered by that frequency
demodulation. The extra circuitry is included to
help block out ambient light and noise to make
the incoming signal more reliable. When a button
is pressed, the IR LED in a remote control turns
on and off in specific timing patterns to
transmit the button press data to the
receiver. The "on times" aren't really fully on
but rapidly turned on and off at the carrier
frequency (38kHz modulation in this example). If
the IR LED "on times" were fully on (like
looking at a flash light), the receiver chip
would think this was ambient light and ignore
it. Since the "on times" are rapidly
fluctuating (modulated) at 38kHz, the receiver
chip's demodulation circuit triggers the latch
circuit and it triggers the data pin. When the
38kHz modulated IR light turns off (the button
on the IR remote control is released), the
receiver chip's demodulation circuit releases
the latch circuit and it releases the data pin.
</p>
<p>
To make things a little more complicated for
hardware building and software programming, the
CIR receiver chip's latch circuit doesn't
respond immediately to any changes. This is done
intentionally to help avoid noise problems and
increase signal reliability. Most receiver
chips' latch circuits won't activate the data
pin until about a dozen pulses at the carrier
frequency are received. When the IR carrier
frequency is stopped, the receiver chip's latch
circuit will also hold the data pin active for
several to a dozen more pulses in length. This
helps make up for the initial delay and will not
give spurious data signals if a few pulses of
the carrier frequency are missed by the
demodulation circuit. The latch circuit working
this way makes for clean data pulses on output.
</p>
<p>
The latch circuit delays will cause problems for
critical timing applications. If the IR carrier
burst is too short (under 12 cycles at 38kHz in
this example), the latch circuit will likely not
trigger its data pin. If the application expects
short signals off the data pin, the latch
circuit may hold the data pin for too long and
cause a timing error again. If the latch circuit
does not release in time for a short space and
another IR carrier burst comes in, the output
data pin will be held high for the entire time
and make those two IR carrier bursts look like
one. Different receiver chips (even of the same
model) also have differently timed lengths for
their latch circuits and may cause slight
variations in the data pin time.
</p>
<p>
The specific demodulation circuitry in the CIR
receiver chip doesn't mean that it will only see
one carrier frequency. It will likely respond to
10-20% higher or lower than the primary carrier
frequency (38kHz in this example), but the
physical distance of that response will decrease
as the frequency shifts away. Frequencies that
are more off will mean that the data pin latch
is less likely to be triggered by the
demodulator. This is why it is important to keep
application frequencies properly matched between
hardware.
</p>
<p>
IR LED transmitters usually don't have frequency
problems since they are such simple and rapidly
responding devices. The transmitters also do not
have to decode data from a potentially unknown
and unreliable source like the receivers do. So
long as the transmitting hardware is properly
clocked and has all its data ready to go, it's
hard to screw up a transmitter. For all the
reasons given so far, this is why some IR
hardware can transmit a short burst protocol but
not receive it (the receiver has to be rated for
the short burst protocol).
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