Austin, TX--While the rest of the world chased higher- and higher-speed modems, engineers at Silicon Laboratories saw a huge untapped demand for low-cost, low-speed devices--units that could bring telecom access to innumerable vending machines, point-of-sale terminals, utility meters, etc.
Such appliances would only need to communicate small amounts of data, (a meter reading or whether the Diet Coke or 93 octane is running low). A low-speed modem to relay such information to a central computer can connect and transmit the data in a few seconds, compared to the tens of seconds needed just to establish a high-speed modem connection-- "Much like a Ferrari beating a jet plane in the first few seconds of a drag race," notes Silicon Labs Application Engineer Jeff Gokingco. The trick was to design a modem to enhance customers' products and do it with such reliability and versatility, at such a low total cost, that its value would be compelling. To that the engineers added making it compatible with globally diverse phone standards, resulting in a formidable design task.
Small thinking. One traditional way to reduce product cost is by cutting the number of components, and if doing so makes the product smaller, it can fit into even more applications. But this is not done so easily in the case of modems, says Tim Dupuis, lead designer for the modem project. "The big problem is isolation--meeting FCC requirements--handling thousands of volts from lightning strikes. Usually this is done magnetically, with transformers, or optically, with opto isolators--big and discrete component technology," notes Dupuis, "which results in large, multicomponent printed circuit boards."
Medical and security systems
Industrial process monitoring
Set-top boxes/Utility meters
Vending machines and ATMs
He adds that another possible isolation component is the capacitor. Usually isolation is not done capacitively, says Dupuis, because "capacitors are difficult to work with" and most designers are more comfortable with magnetics. "The problem with capacitors in modem applications is that they do not conduct low frequency well. As modem frequencies go down to 200 Hz, you would need a large capacitor, or else go with a familiar transformer."
But Dupuis' team had some engineering legerdemain up their sleeve--the company's patented ISOcap™ technology, which uses two CMOS chips to pass data and control signals across a capacitive isolation barrier digitally. The digital data processing allows a smaller capacitance, whereas competitive capacitive technology uses larger components because of analog processing. "By using modulation data processing technology at MHz levels in an analog-to-digital converter, we can get away with a small capacitor," highlights Dupuis.
The two-chip solution for the resulting ISOmodem™ produced a highly integrated device, cutting the need for large discrete components such as a transformer and relay--resulting in a 60% drop in power required, a 75% reduction in board space, and an overall cost savings of 40%.
Of equal importance was use of the company's integrated data-access arrangement (DAA), a programmable telephone-line interface that meets global requirements. Whereas discrete component modems have fixed circuits (which only meet one telephone standard, thus dictating a separate product for each market), only one ISOmodem need be produced and inventoried. All worldwide phone parameters can be programmed into a look-up table, so that the only thing a vendor need do is tell the software the customer's location which, via the initialization code, sets the proper registers. The ISOmodem also supports such functions as decoding caller ID while in its powered down mode, something fixed circuits cannot do.
But Silicon Labs engineers also took into account the need for potential design changes in the future. The two chips incorporate a single metal layer ROM, which is the topmost layer. Any code changes can readily be made to this final mask step in the fabrication process with minimal production disruption.
The all-silicon ISOmodem is designed for low-speed(&2,400 bit/sec). The two chips also include the audio coder/decoder (codec), digital signal processor (DSP), and microcontroller (in the Si2400 chip interfacing to the host application) and analog front end (in the Si3015 chip on the phone line side) (see figure). A single-chip solution is not possible due to the lightning isolation requirements. The ISOmodem is geared to widespread embedded modem uses by virtue of its small board space, low power needs, and global compliance.
Tools of the trade. While the ISOmodem's design is unique, it was brought to market in only eight months. Here expertise, skill, and serendipity all played a part. "A key factor," according to Dupuis, "was that the low-speed requirement allowed designers to develop a software emulator for the ISOmodem able to run in real time on a PC with 500 MHz processors. A high-speed modem, like a V.90, couldn't be emulated on a PC."
Dupuis notes, "The big challenge for a modem is to get the algorithms developed and tested. We had the DAA under control, and our designers could write the code, and develop the single DSP core and microcontroller optimized for this mixed signal application."
In getting to the emulation stage, Dupuis gives credit to Design Compiler software from Synopsys (Mountain View, CA). This logic compiler, used for the ISOmodem DSP and microcontroller, allows a designer to write in VHDL and outputs the logic for the application. Silicon Labs engineers then did the chip layout using Silicon Ensemble by Cadence Design Systems (San Jose, CA).
Dupuis notes that in development, the designers acquired an even greater appreciation of how severe lightning can be on modems. "While FCC standards require testing to 1,500V, phone line primary protection boxes are supposed to limit voltages to 600V. But many of these are faulty or worn out." Silicon Labs tested the ISOmodem to 7,000V between the modem and the phone lines. "This is not a PC modem," emphasizes Dupuis. "It will be used in security and remote applications and can't be replaced readily, so we use higher rated capacitors to ensure operation." The company did on- and off-hook testing with lightning pulse generators to "understand where the current goes," says Dupuis. He adds that using a Tektronix (Beaverton, OR) battery-powered (of course) oscilloscope was vital in measuring currents during strikes.
Finally, the value of testing was further realized, says Dupuis, when it was found that the ISOmodem had difficulty in connecting with some older modems. "You have to test," he adds, "since even if you meet modern specs, you still won't necessarily work with all others" not built to those standards. "We ended up tweaking the code for older modems."
Based on design acumen, coupled with designers listening to customer wants, Silicon Laboratories ISOmodem is two to four times cheaper than the competition, while providing more features in a universal, programmable package.
For more information on ISOmodems, Contact Tim Dupuis, Silicon Laboratories, 4635 Boston Lane, Austin, TX 78735; Tel: (512) 464-9321; Fax: (512) 464-9444; E-mail: firstname.lastname@example.org.