AN4099 datasheet(6/39 Pages) STMICROELECTRONICS

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Infrared protocol specification

AN4099

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Doc ID 023110 Rev 2

Infrared protocol specification

1.1

RC5 protocol basics

The RC5 code is a 14-bit word that uses bi-phase modulation (also called Manchester

coding) of a 36 kHz IR carrier frequency. All bits have an equal length of 1.778 ms, with half

of the bit time filled with a burst of the 36 kHz carrier and the other half being idle. A logical

zero is represented by a burst in the first half of the bit time. A logical one is represented by

a burst in the second half of the bit time. The duty cycle of the 36 kHz carrier frequency is

33% or 25% which reduces power consumption.

Figure 1.

RC5 bit representation

The RC5 frame can generate 2048 (32 x 64) different commands organized in 32 groups.

Each group has 64 different commands. An RC5 frame contains the following fields. An

example of an RC5 frame is shown in Figure 2.

●

Start bit (S): 1 bit length, always logic 1.

●

Field bit (F): 1 bit length, which denotes whether the command sent is in the lower field

(logic 1 = 0 to 63 decimal) or the upper field (logic 0 = 64 to 127 decimal). The field bit

was added later when it was realized that 64 commands per device were insufficient.

Previously, the field bit was combined with the start bit. Many devices still use this

original system.

●

Control bit or Toggle bit (C): 1 bit length, which toggles each time a button is pressed.

This allows the receiving device to distinguish between two successive button presses

(such as "1", "1" for "11").

●

Address: 5 bits length that selects one of 32 possible systems.

●

Command: 6 bits length that (in conjunction with the field bit) represents one of the

128 possible RC5 commands.

889 μs

Logical ‘0’

Logical ‘1’

889 μs

ai18530

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