Issue 138 January 2002
WHAT GOOD IS IrD,Eh?
PART 1: CORDLESS PROTOCOL

byJeff Bachiochi

Start • Standards Make It Happen • IrPHY • The Range • UART To IrDA • IR Encoder/Decoder • Gimme More • Sources & PDF

IrPHY

To conform to IrDA standards, the devices used must meet certain criteria. Let’s look at the requirements to see what’s most important. The medium for data transmission is infrared with a peak wavelength between 850 and 900 nm. On the transmission side, an IR LED is a directional device, meaning the plastic package forms an integral lens. The lens confines its radiant output into a conical beam pattern to either concentrate it in a narrow beam (small angle) or spread out over a wide beam (large angle).

IrDA defines the area of interest to be a beam width of no less than 30°, ±15°. Maximum and minimum intensity specifications are therefore defined within this beam width. This specification assures communication by accepting a certain amount of misalignment between communicating devices while preventing radiated energy from potentially interfering with other devices within the same room.

The minimum intensity is required to assure a connection at the maximum distance. For standard IrDA, that distance is 1 m. A newer low-power specification reduces the distance to about 30 cm. Maximum intensity eliminates flooding the room with interference. Additional requirements include minimum rise and fall times, pulse widths based on the data rate, and edge jitter tolerances.

On the receiver side, the IR sensor must have at least the same 30°, ±15°, cone of reception to accommodate misalignment of a communicating device. The receiver must be sensitive enough to receive IR pulses of the minimum intensity at the maximum distance of 1 m (for standard devices). While at the minimum distance, the IR receiver must withstand becoming saturated by a bombardment of IR from the transmitter’s maximum intensity. To cover this wide range of intensity, some receivers contain a kind of gain control. Specifications require the receiver to return to maximum sensitivity within a specified time, which is based on whether the device is of the standard or low-power variety.

IrDA specifies a bit error ratio (BER) of not more than 10–8 (one in 100 million). If the transmitter or the receiver doesn’t live up to the specifications, then a dropout in communications could occur.

The specification could have allowed TTL serial data to directly couple to the IR transmitter and receiver. However, that would require the maximum transmitter current to be sustained over the full bit time. This would necessitate large power requirements because current can be high through the transmitting LED to get maximum radiance. Many portable devices use the IrDA protocol, so battery life is a serious issue. In order to reduce transmitter current to a minimum, the specified IrDA IR pulse width is significantly shorter than a bit time. Table 1 shows the details.

Figure 1—The upper plots shown here show UART transmission and the associated IRTX pulses produced. The lower plots show how higher data rates using synchronous data combine bit pairs to produce a single time slot IR pulse.

Devices sending data at 115.2 Kbps or slower use an asynchronous format in which an IR pulse is transmitted for each space (zero) bit time (see Figure 1). Signaling rates from 576 Kbps to 1.152 Mbps use a synchronous format in which an IR pulse is transmitted for each space (zero). Data rates of 4 Mbps use a synchronous format in which each two bits are translated into a single pulse during one of the four 0.5 bit times, which make up the 2-bit time period.