May 1999, Issue 106

Dallas 1-Wire Devices, (Part 2):
All on One

THE BUS STOPS HERE

Park those buses in an infinite loop for now. I want to continue where I left off last month, talking about the 1-wire Dallas devices. In their play for easy expansion, Dallas created a potential monster by enabling multiple 1-wire devices to share the same single I/O bit.

Many of these devices might be the same, like the multiple thermometers that monitor equipment temperatures, or they might be different, like the 1-wire weather station Dallas offers for about $80 on their web site (see Photo 1).

The plastic enclosure contains the electronics and hardware for measuring temperature, wind speed, and wind direction. All of this is done via Dallas 1-wire devices. How is it possible, using a single wire?

There are two levels of commands to talk with 1-wire devices—ROM functions and memory functions. All 1-wire devices are familiar with all ROM functions, which are bus-level functions used to communicate with all 1-wire devices. (See Part 1 for 1-wire timing and communication specs.)

Following a bus reset, all slave devices patiently listen for a ROM function. After a device is selected via a ROM function, it is ready to respond to a memory function. Each 1-wire device family has its own list of memory functions that support the special qualities of that device. In other words, ROM functions are general commands and memory functions are specific.

The READ-ROM command instructs all devices to forget about the ROM functions and respond with their ID (family code + serial number + CRC). Obviously, multiple devices will answer and the colliding data is worthless, making READ-ROM only good for use with single devices on the bus.

SKIP-ROM instructs all devices to forget about the ROM functions and respond to the following memory function. Again, good for single devices.

MATCH-ROM contains a unique ID (family code + serial number + CRC) of the device you’re looking for. All 1-wire devices that don’t match the ID you requested are disabled. Only a single device (if it is actually there) pays attention to the following memory function commands. MATCH-ROM is good to use with multiple devices—that is, if you know who’s connected.

56-BIT ADDRESSING

If you paid attention last month, you know how each 1-wire device has its own ID number. Each device has its own 1-byte family code (e.g., 10h for all DS1820 thermometers). The following six bytes hold a unique serial number for each manufactured device. No two devices within a family have the same serial number.

A single-byte CRC check byte offers some semblance of correctness. To determine what devices are connected to the 1-wire bus, you must search all 264 combinations to see who’s home. But, that task could take years! Luckily, Dallas included a search function to help narrow down the possibilities, but you need to know how to use it.