Design Feature: April 11, 1996

Last year, the maximum Infrared Data Association (IrDA)-sanctioned transfer speed for wireless-IR devices was 115.2 kbps (Reference 1). IBM offered 1.152-Mbps devices that complied with an industry-proposed spec, and IrDA was considering a 4-Mbps protocol. The 4-Mbps speed is now a reality; IrDA 1.1 adds high-speed extensions to the 1.0 norm, IrDA-SIR (serial) to cover 1.152 and 4 Mbps, IrDA-MIR and -FIR, respectively (medium and fast IR).

The 35-fold leap in speed promises to be a significant factor in system designers' decision to incorporate wireless-IR utilities into equipment. Transferring large files at 115.2 kbps can consume a great deal of time. At this rate, transferring a 10-Mbyte file takes 1000 sec—almost 17 minutes. At 4 Mbps, the transfer takes 30 sec.

Before the advent of the 4-Mbps extension, wireless-IR data transfer was mainly a matter of convenience—an easy way to dispense with cables. Now, the higher rate makes IR transfer one of the fastest ways to move data. Table 1 gives the data rates of various transfer methods. Only an enhanced parallel port matches IrDA 1.1's 4-Mbps transfer rate.

The IrDA protocol

The IrDA protocol stack covers the physical transfer of data bits, guidelines for link access (IrLAP), and link management (IrLMP) (Figure 1). IrLAP manages the software that seeks other machines to connect to, or "sniff," discovers other machines, resolves addressing conflicts, initiates a connection, transfers data, and disconnects (Figure 2). IrLMP assesses the equipment and services available to connected equipment and negotiates data-transfer parameters.

In the physical layer, a pulse-width modulator encodes a bit stream from the transmitting equipment's UART. The encoder keeps pulse width less than 3/16 of the bit-rate period, mainly to minimize power dissipation in the transmitting LED. The edge detector and pulse-width demodulator in the receiver section must decode, or stretch, the pulses in the incoming bit stream, for presentation to the UART in the receiving equipment.

The 1.152- and 4-Mbps modes require some changes in the transceiver circuitry and driver software. The major hardware difference is the need for a dedicated communications controller. Standard UARTs could directly interface transceivers at the previous 115.2-kbps data-transfer rate but cannot do so at the higher rates. The communications controller provides monitoring and data-flow control between the ISA-bus FIFO device and the UART FIFO device and allows direct access to the computer via the external bus.

To accommodate the increased data rates in the high-speed extensions, IrDA 1.1 shortens the relative pulse width, thereby reducing power consumption, and mandates a synchronous data channel, as opposed to the asynchronous channel IrDA 1.0 used. The extensions begin a transmission with a frame flag and transmit the rest of the frame without start and stop bits for each character. The protocol dictates an end-of-frame signal and an appended cyclic-redundancy-check (CRC) word. The 4-Mbps mode achieves its speed by using pulse-position modulation (PPM) to encode 2 data bits per light pulse. It thus yields four possible time-slice divisions per data symbol.

Cost is the reason for selecting 4 Mbps as the upper limit for IrDA transfers. Higher frequencies would entail the use of much more expensive LEDs and photodiodes. The high-speed modes are transparent to IrLAP and IrLMP; IrLMP negotiates speed options during its normal discovery process. All IrDA-MIR and -FIR products are backward-compatible with IrDA-SIR products and protocol layers.

You have a choice of several types of hardware products for effecting IrDA-protocol wireless transfers. Table 2 gives a representative listing of recent IrDA-compliant products. First, for laptop or desktop computers lacking IrDA capability, you can connect a tethered adapter, or "dongle," to the computer's serial port. The PhasIR from Amp, for example, comes with a 2m tether and a nine- or 25-pin RS-232C connector. Because it interfaces with the computer's standard UART, the data-transfer rate is limited to IrDA-SIR: 115.2 kbps. For rates as high 4 Mbps, Amp offers custom versions of the adapter for connection to a parallel port.

The second category of IrDA-compliant products is transceiver modules. These devices integrate the transmitting LED, receiving positive-intrinsic-negative (PIN) photodiode, and receiver/transmitter IC into a small, molded package. The modules from Hewlett-Packard, IBM, Siemens, and Temic accommodate IrDA-SIR, -MIR, and -FIR; the Hewlett-Packard HP-SIR mode; and the 9.6-kbps Sharp (Mahwah, NJ) amplitude-shift-keying standard that many personal digital assistants use (Table 2).

IrDA modules typically use differential circuitry for the receiving photodiode (Figure 3a). This circuitry provides high interference immunity by effectively isolating the diode from such villains as power-supply fluctuations and ground bounce. Most of the power dissipation in the modules is the power consumed by the transmitting LED, which typically draws 150 mA (average) and 600 mA (pulsed).

In a connection diagram for the IBM and Temic modules, the highpass network on the transmit (TxD) pin guards against overlong transmission signals (Figure 3b). The shutdown (SD) pin assumes a dual role: First, in pure shutdown mode, the module draws less than 75 mA. Second, with certain programming sequences on the SD and TxD pins, you can choose either a 9.6-kbps to 1.2-Mbps operating range or a 4-Mbps operating range.

The next class of IrDA-compliant devices available for OEM design-in is transceiver ICs that lack the transmitting LED and receiving photodiode. These devices are ASICs that contain all the circuitry inherent in the transceiver modules. They're useful in applications in which you need to obtain particular optical characteristics. The modules that incorporate the LED and photodiode have tightly defined specs on optical viewing angle and dispersion, for example. You may need to design a system with particular optical characteristics, in which you custom-tailor the placement of and lens system for the optical components. One such transceiver, IBM's 31T1101, incorporates the electronics for the company's 31T1100 transceiver module (Table 2).

Receive-only ICs can be useful and economical in systems that incorporate an IrDA-compliant controller. These ASICs, available from Irvine Sensors and Unitrode, require only the connection of a PIN photodiode. Figure 4 shows the simple connection of Irvine's SIRF IrDA-SIR and -FIR receiver. For both its receiver ICs, Irvine claims a wide dynamic range that allows reception at distances as great as 3m (IrDA specifies 1m maximum). A shutdown pin in the SIRF reduces operating current to 10 mA.

These transceiver and receiver devices would be of little use if they had no effective means of interfacing with the host equipment. Several products do provide the means, however. For example, Hewlett-Packard's HDSL-7000 does the encoding and decoding for the company's HDSL-1000 transceiver module and provides an interface with a standard 16550 UART. Its functions include pulse shaping (to 3/16 of a period) and stretching in compliance with IrDA 1.0.

Similarly, IBM's 31T1502 IR communications controller provides an ISA or PCMCIA interface to the company's 31T1100 transceiver module and to other IR transceivers. Intended for design-in on ISA boards or PC cards, the 31T1502 offers host-DMA and shared-memory modes and an advanced interrupt capability that provides independent routing of the IC's UART and FIR-controller internal interrupt lines.

Intended for embedded applications such as printers and modems, National Semiconductor's PC87108 provides 16550 compatibility to support existing legacy software (Figure 5). The device supports all IrDA modes, Sharp-IR, and consumer-IR (TV remote). Computer-interface aspects include ISA compatibility, interrupt routing to one of seven output pins, DMA-handshake signal routing for either one or two channels, and power-management support.

Last year, the densely packed, IrDA-compliant multifunction chips from National Semiconductor (dubbed Super-I/O) and Standard Microsystems (dubbed Ultra-I/O) seemingly offered every conceivable function but rinse and spin-dry cycles. Recent multifunction chips from both suppliers support the IrDA-MIR and -FIR high-speed extensions. National's PC87308 and PC87338 and Standard Microsystems' FDC37C93XFR each have a floppy-disk controller, two UARTs plus IR support, a parallel port, Plug-and-Play support, and power-management facilities.

The three chips differ in a few respects. National's PC87338, intended for portable computers, dispenses with the keyboard (and mouse for the PC87308) controller and the real-time clock that the other two devices have. The PC87338 is designed to work with most portable chip sets, such as those from Intel (Santa Clara, CA), Opti (Santa Clara, CA), and PicoPower (Fremont, CA), and with 486, Pentium, and PowerPC µPs. It operates in 3.3, 5, and mixed 3.3/5V systems.

Finally, it wouldn't be appropriate to neglect the software aspects of IrDA wireless communications. Windows 95 includes drivers for IrDA support. The general-purpose notebook-to-desktop LapLink from Traveling Software supports the Microsoft drivers, as does the IrDA-dedicated TranXit from Puma Technology.

You can reach Senior Technical Editor Bill Travis at (617) 558-4471, fax (617) 558-4470, e-mail b.travis@cahners.com

Reference

1. Travis, Bill, "Ease transfers with IrDA-protocol wireless infrared," EDN, Mar 30, 1995, pg 59.