Issue 150 January 2003
GUI Interfacing
A Straightforward, Simple Solution

iBUTTON

Dallas Semiconductor has been supporting iButtons and one-wire technology for several years. One-wire technology that’s housed in a stainless steel case has proven to be a safe, durable, and inexpensive approach to identification, data storage, and security.

The DS1963S iButton has partial storage protected by a secure hash algorithm (SHA-1) system. A secure secret written to a write-only (WO) register is used (along with many internal registers) to calculate a message authorization code (MAC). The unreadable MAC is checked internally against a user-entered MAC. A match indicates a trust that data is not tampered with. Again, a detailed description of security features is beyond the scope of this article and can be revisited in a future column. 

For the purposes of this project, I directly altered a 2-byte location within the first page of an NV memory at addresses 0x18 and 0x19. The user credits are stored at these addresses for clarity and simplicity, allowing the code to be followed easily. Note that without the use of the special mechanisms afforded by the secure iButton devices, you’re leaving the system open to counterfeiters.

Dallas Semiconductor has a cool IC that makes it easy to add one-wire communication to a system that has a serial port. The DS2480B can take serial commands and handle all of the one-wire communication in the background. This device works well in a dongle connected externally to a serial port. Also, it has inverted logic for connecting directly to the TTL serial connections on a microprocessor (see Photo 4).