Maxim Blog | Behind the Scenes — The Ubiquitous iButton

By Christine Young, Maxim Integrated blogger

There are now more than 20 products in the iButton family, designed to transfer or record data where no other product can. Devices track temperature and humidity, provide access control and asset tracking, and enable electronic cash transactions. After doctors from local university hospitals visited Maxim's headquarters in San Jose to learn about iButtons in corrections, I wanted to learn more about the origins of these tiny devices. So I talked to Hal Kurkowski, one of the inventors of the iButton and currently executive director of Maxim's Microprocessor and Security Products Division.

The story begins at Dallas Semiconductor (acquired by Maxim in 2001), whose product line included nonvolatile static RAM. In traditional techniques for storing data, write sequences often take longer than read sequences, which can be problematic for certain types of applications. Drawing on products developed by its founder, Dallas Semiconductor engineers created battery-backed static RAM, in which read and write sequences are synchronized; battery power maintains data during periods when the chip is powered off. An offshoot of this RAM product, a portable token, became a forerunner of iButtons and smart cards. The token functioned similarly to today’s USB dongles, allowing it to exchange data with a host computer. Designers used pins from a dual in-line package (DIP) to provide an electrical connection to the chip inside the token. But Kurkowski explains that these pins were very fragile, which limited the usefulness and reliability of the token.

Around the same time, the company was developing the 1-Wire® serial protocol, which provided low-speed data, signaling, and power over a single conductor. Integrating the 1-Wire protocol into a token brought it closer to today’s iButton. But engineers had to address reliability issues. At the time, they looked at the lithium-ion coin cell battery industry. To communicate with the chip inside the token, at least one contact and one ground were required, much like a battery. So, the engineers thought: What if we took the chip out of the token and put it in a coin cell battery housing? Engineers mounted a tiny PCB containing the chip into a stainless steel housing to provide a more rugged enclosure. The 1-Wire protocol provided a way to communicate with and power the chip in the coin cell battery housing. For products requiring nonvolatile read/write memory, a real-time clock, or a temperature data logger, a small battery was added to the housing to provide the necessary onboard power.

Data captured by iButtons can be transferred to a computer for inspection and analysis, and the devices can be mounted on any object.

As Kurkowski points out, there are still many areas where the iButton needs to be enhanced, because it is very difficult to attach anything to the smooth button-cell-style housing. Double-sided tape or glue is not practical for a device that may last up to 10 years and be used in harsh environments. The engineers decided to design a flange around the bottom of the housing and a mechanical crimp ring on the front, which would mechanically fix the iButton to the end product. "Now we can integrate any function we want into the chip, it can be a memory chip, a real-time clock, a temperature measurement chip," Kurkowski added.

As iButton developers created different versions of their products, they kept the form factor consistent for ease of use. A set of probes and readers are available to support any type of iButton. Different parameters, such as data logging frequency, are easily set by the user, and data can be downloaded to a computer through an app. Thermochron iButtons are temperature-measuring devices that come with a chip, a battery, and a real-time clock (RTC). From an engineering perspective, the iButton Hygrochron temperature/humidity logger is even better. iButtons are usually completely sealed, but this iButton needs access to the surrounding air. To provide access, engineers designed a tiny hole in the top of the button, mounted the humidity sensor just below, and sealed the rest of the device to protect the other electronics, Kurkowski said.

“One of the advantages of the iButton is that it has very little thermal mass, so the response time is very fast,” Kurkowski said. “Also, the device can last for 10 years or a million temperature conversions — whichever comes first.”

Obviously, the invention of the iButton required a certain amount of ingenuity and creativity, as well as the ability to bring together separate engineering designs. The engineering team also needed to look at work in other areas, such as button batteries. Since iButtons have a similar shape and size, the team was able to leverage battery manufacturing technology to produce their device.

“In a lot of engineering, what you need to do is look around to see if someone has already solved the problem, because you have a lot of problems to solve,” Kurkowski said. “You want the end product to be very unique and creative and hard for others to copy. But on the other hand, you don’t want to invent everything from A to B, so you need to see what difficulties have been overcome by other engineers.”

iButton has a wide range of applications, and there is nothing that cannot be done.

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