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As shown in Fig. 13, the bi-phase code words modulate a 36Khz carrier before being transmitted via
the IR LED. The 36Khz carrier renders the transmission immune to TV line scan interference. Since
the repetition period of the 36Khz carrier is 27.778µs and the duty factor is 0.25, the carrier pulse
duration is 6.944µs. Since the high half of each symbol (bit) of the RC5 code word contains 32 carrier
pulses, the period is 64 X 27.778µs = 1.778ms. and the 14 symbols (bits) of a complete RC5 code
word take 24.889ms to transmit. Once a transmission has been initiated, a complete code word is
sent, even if the key is released. The code word is repeated every 113.778ms (4096/f
a key remains pressed. Each 14-bit RC5 code word is preceded by a de-bounce time followed by a
keyboard scanning time. A code word consists of:
• A start bit (S) which is always logic 1.
• A field bit (F) which denotes command codes 0 to 63 (logic 1) or command codes 64 to 127 (logic 0).
Logic 1 is used.
• A control bit (C) which toggles after each key release and initiates a new transmission.
• Five system address bits for selecting one of the 32 possible systems. System no 0 is selected
(TV receiver 1).
• Six command bits representing one of the 128 possible RC5 commands. Ex.: command 32 is
program up and command 33 is program down.
The Z-421 infrared receiver demodulates the signal (remove the 36 KHz carrier), amplifiers and inverts
it. The demodulated remote control bit-stream is fed directly to the microcontroller for decoding.
24.889ms
1.778ms
S
F
C
1
1
0
6.944 us
27.777 us
113.778ms
5 system bits
0
0
0
0
1
32x
Fig : 13 : Transmission details
6 command bits
1
0
1
1
21
3DQDVRQLF
) as long as
carrier
1
1
S = Start
F = Field bit
C = Control bit
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