Use of IR (infrared) light as a method for wireless communication has become popular
for remote control applications. There are a number of different standards for such
communication. In this application note the widely used RC5 coding scheme from
Philips will be described, and a fully working remote control solution will be presented.
This application will use the ATtiny28 AVR microcontroller for this purpose. This powerful unit contains a hardware modulator, a high current LED driver and interrupt
options which makes it especially well suited for these kinds of applications.
Click here to download the full size of the above Circuit.
Utilizes ATtiny28 Special HW Modulator and High Current Drive Pin
Size Efficient Code, Leaves Room for Large User Code
Low Power Consumption through Intensive Use of Sleep Modes
Cost Effective through Few External Components
The RC5 code is a 14-bit word bi-phase coded signal (see Figure 2). The two first bits
are start bits, always having the value 1”. The next bit is a control bit, which is toggled
every time a button is pressed on the remote control transmitter. This gives an easy way
of determining whether a button is pressed and held down, or pressed and released
continuously. Five system bits hold the system address so that only the right system
responds to the code. Usually, TV sets have the system address 0, VCRs the address 5
and so on. The command sequence is six bits long, allowing up to 64 different comm a n d s p e r a d d r e s s . T h e b i t s a r e t r a n s m i t t e d i n b i - p h a s e c o d e ( a ls o k n ow n a s
Manchester code) as shown in Figure 3. An example where the command 0x35 is sent
to system 5 is shown in Figure 4. Note that Figure 3 and Figure 4 show the signal that
enters the ATtiny28 hardware modulator. The actual signal emitted by the IR-LED will
be modulated with a certain carrier frequency as shown in Figure 5.
ATtiny28: The Ideal
ATtiny28 is a low cost,...
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