Functional Overview

2.3Infrared methodology

Most infrared control systems only need to transmit small data rates, but a vital prerequisite is safety of transmission such that unintelligible signals must simply be ignored. Commands can be transmitted by digital transmission employing modulation of the infrared optical carrier. There are numerous standards established and the PrimeCell GIR is capable of implementing most of them.

Commands may be transmitted as a code word where data are defined as a series of bits or binary values.

There are various methods of encoding that may be employed to represent each bit as a symbol to be transmitted, and the PrimeCell GIR requires this encoding and decoding to be performed in software. Three commonly used representations of a bit in infrared remote control systems are as follows:

•Frequency Shift Keying (FSK) transmits a bit using two different frequencies. Although this is accepted as a safe method, a major disadvantage is cost and power consumption.

•Biphase modulation (Manchester encoding) represents a bit using an edge transition, where a rising edge represents 1 and a falling edge represents 0. The symbols are modulated by a carrier frequency.

•Pulse distance modulation is used with a carrier and also in a pulse modulated mode (also called flash mode). This is where distance between the rising edges defines the bit to be 0 or 1, for example, a short distance between two pulses represents a 0 and a long distance represents a 1.

Functional Overview

Frequency Shift Keying is illustrated in Figure 2-2.

Frequency Shift Keying (FSK) is coding of bits using two frequencies,

= frequency f1 = frequency f2

The sequence of the transmitted frequencies determines logic 0 or 1.

For example:

Figure 2-2 Frequency shift keying

In Figure 2-3 biphase encoding is shown where a rising edge within a time window represents a logic 1 and a falling edge denotes a logic 0.

Bi-phase encoding (Manchester encoding) represents a bit using an edge transition. For example

bit period

Figure 2-3 Biphase encoding

Functional Overview

Pulse distance modulation is illustrated in Figure 2-4.

Pulse distance modulation represents a bit by the distance between edge transitions. For example:

Figure 2-4 Pulse-distance modulation

A commonly used standard in Europe is RC-5 which uses biphase encoding and modulation by a 36kHz carrier.

The transmission of a code word begins with two start bits (start bit, S and field bit, F) followed by a toggle bit (or control bit, C) that changes value in each code word (this distinguishes interruption of the infrared link from multiple key presses). The remainder of the code word consists of a 5-bit address and a 6-bit command.

The high phase of each bit in the code word consists of a burst of 32 pulses with a repetition rate of 36kHz. The timing is shown in Figure 2-5 on page 2-10.

Functional Overview

113.778ms

24.889ms

6.944 s

27.778 s

32 Carrier Pulses

Figure 2-5 RC-5 code word transmission

With a 36 kHz carrier, the period is 27.778μs and at a duty cycle of 0.25 the carrier pulse duration is 6.944μs. Since the HIGH half of each symbol (bit) in the RC-5 code word consists of 32 carrier pulses, the symbol period is 64 x 27.778μs = 1.778ms and the 14

symbols (bits) of a complete RC-5 code word take 24.889ms to transmit. A RC-5 code word is repeated every 113.778ms (4096/fCARRIER) as long as a remote control key remains depressed.