September 2004, Issue 170

Multilab
Build a Z8 Encore!-Based Multipurpose Test Instrument

IrDA and RS-232 LINKS

The design of the IrDA link to the Palm Pilot is simple, thanks to Zilog. The Z8 Encore! contains two UARTs, each of which includes a complete IrDA encoder-decoder, or ENDEC. The IrDA physical layer encoding/decoding isn’t a complicated process. The data format is basically the same asynchronous format used for RS-232 serial communications, except that instead of using two different voltage levels to represent ones and zeros, the IrDA format uses a short pulse within the bit cell time to indicate a zero and the lack of a pulse to indicate a one.

In asynchronous data protocol, all characters start with a 0 start bit; therefore, all IrDA characters start with a short pulse representing this start bit. This forms the timing reference needed to recover the following 8 data bits, as well as the stop bit at the end, which is a one.

The pulse width used by IrDA is defined as one-sixteenth of the bit cell time (the reciprocal of the data rate). It’s the timing of this pulse width that creates a bit of a problem. In an MCU that contains just a standard UART function, adding the ENDEC function outside the MCU can be done using specialty chips designed for this purpose, like the Texas Instruments TIR1000 and Unitrode UCC5343. However, these devices require a timing reference at 16 times the data rate used by the MCU’s UART.

This timing signal, although definitely present within the MCU’s UART block, is generally not brought out to any of the MCU’s port pins. Therefore, it has to be provided for in some other way. That’s why the ENDECs contained within the Z8 Encore! are so handy.

I used the Z8 Encore!’s UART1 for the IrDA port. To switch from normal asynchronous data protocol to the IrDA protocol, all you need to do is set the U1CTL1 register’s bit 0 to 1. The only hardware needed is the IrDA transceiver module, which contains both the IR LED/driver and detector/amplifier. During early tests, I was impressed with the operation of the Zilog IrDA transceiver module contained on the evaluation board, so I used a similar ZHX1820 transceiver module for this project.

The circuitry only handles the physical layer of the communications link. There is an extremely sophisticated (meaning lots of code!) protocol called irCOMM that PDAs, laptops, and printers use to communicate with each other in a wireless fashion. It handles all of the nitty-gritty details of networking these devices together in an error-free way.

The Palm Pilot includes all of the drivers for the protocol in its operating system. I did not come across an irCOMM protocol stack for the Z8 in the C libraries I had. When I looked at the protocol’s details, I decided it was much too complex for my application.

IR communication has some parallels in RF communication. Stray noise found in RF is replaced with stray light. In both cases the distance between the transmitter and receiver may cause the received signal strength to vary over a wide range. The IrDA transceiver modules do all they can to combat this. The IR LED is pulsed fairly hard to produce a strong IR light signal, and an AGC-controlled amplifier follows the photodector to even out the differences in received signal strength.

In my experiments with a Palm Pilot and the Z8 Encore! development board, I found only a few things that you need to worry about when trying to achieve error-free data transfers between the two devices. First, note that you have to keep the two devices pointed at each other at a distance of less than one yard. Second, allow a fraction of a second of dead time between transmitting and receiving data because the IrDA link is only a half-duplex link. Third, send a short preamble before the actual message because the IrDA receiver operates using AGC. The preamble gives the receiver time to adjust from a state in which it looks (in a sensitive way) at stray light to a state in which its gain has been adjusted to match the level of the incoming IR signal. I chose a 10-character preamble of 0xFF bytes before the actual message. As a result, the byte pattern sends one short start pulse per character (the start bit) with which the receiving UART easily synchronizes.

Finally, frame the message to start with a start-of-text control character and end with an end-of-text control character. This allows the receiving end to easily identify the actual message. Using this simple scheme, it’s easy to transmit data back and forth between this design and the Palm Pilot that controls it.

I used UART0, along with a MAX232 for level shifting, for the RS-232 link to the PC. Although the preamble and message protocol used for the IrDA link isn’t necessary for this direct-wired link, I used it here as well to simplify the program.

Fortunately, the MobileVB software I used to write the Palm Pilot application supports both RS-232 and IrDA communications links. It also provides for both raw, which I used, and irCOMM protocols for the IrDA link.