iButtons

Dallas Semiconductor has a group that makes the iButton. The primary interesting things about iButtons are:

a basically indestructable metal case, looks like a thick watch battery. Two contacts: data and ground.
A signalling protocol that can communicate bidirectionally over two wires (the so-called "One-Wire Protocol"), in which the iButton steals power from the data line, and drives it with an open-drain gate.
An identification scheme whereby each iButton has a unique 48-bit ID number, letting multiple devices reside on the One-Wire Bus at the same time. The protocol lets you discover what devices are on the bus, and allows you to talk to a single device at a time.

The basic iButton is just that. It contains a serial number and nothing else. The more interesting models contain various amounts of EPROM, EEPROM, NVRAM (Dallas has a lot of experience at putting lithium batteries in chips), clocks, temperature sensors, small microcontrollers, and Java virtual machines. They have solder-mount versions of many of the iButtons too, TO-92-shaped gadgets that act just like an iButton but are easier to solder down to a board. The serial-number-only device could be useful for assigning ethernet addresses; the temperature device is useful for measuring temperatures.

There is supposed to be a crypto button that is basically a 8051 microcontroller running a special program that will let you run cryptographic scripts, but it isn't clear that you can actually buy one: the online documentation is confusing at best. There is also a Java button that has 6kb of NVRAM and a complete Java Virtual Machine in it. You can buy these: I have one. The Javabutton is supposed to be revised (perhaps next year?) to more than 128kb of NVRAM and will have a useable 1024-bit hardware exponentiator, making stuff like RSA much faster.

This page is meant to document the stuff I find out about iButtons and the hacking I do with them.

Contents:

Kits: I've got the iButton Starter Kit and the JavaButton Kit. I couldn't figure out how much they were or what came in them before I bought them, so I list the prices and contents here for the benefit of others.
My iButton hacking: iButton things I've done. At this moment this only includes a Perl module to talk to iButtons and notes on what I plan to do with it.

Useful iButton sites:

http://www.ibutton.com: The official iButton site, set up by Dallas. Mostly marketing stuff.
http://www.dalsemi.com: Dallas Semiconductor. Data sheets and app notes for iButtons, as well as other fun gadgets.
http://sof.mit.edu/ibuttonpunks/: iButtonpunks, folks that like to hack on iButtons. There's a mailing list. That's about it. Most of the links on this page are teasers or placeholders and don't actually lead to anything.

Miscellaneous notes about iButtons:

Dallas makes it really easy to order these widgets. The easiest thing that I found was to call 1-800-336-6933 and buy stuff with a credit card. You can buy small quantities (<$200) of anything that Dallas sells this way. You can also just ask them for prices in unit quantities. I spent a long time searching various distributors' web catalogs looking for prices with little luck, and I should have just called Dallas. I think this is cool; I've ordered parts from distributors for personal projects, and the reps aren't generally interested unless you're going to be ordering 4 zillion parts over the next year. I can't blame them, I know that I don't represent much in the way of commission, and I'd be happy to go to a mail-order or retail outlet, but a lot of the parts I'm interested in just aren't going to be at Digi-Key or Radio-Shack. So I think it's really convenient that you can order hobbiest quantities straight from Dallas.

Brian Warner <warner@lothar.com>

Last modified: Mon Jan 11 00:12:50 PST 1999