May 1999, Issue 106

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

OVERDRIVE

When it comes to speed, the 1-wire bus won’t win any races. It wasn’t designed for speed. It was designed as a minimal cost interface, drawing minimal current while withstanding large mechanical stresses, with built-in error checking for data transfer reliability.

But, these people couldn’t leave well enough alone, so they created devices capable of overdrive. Overdrive is a special timing specification that enables particular devices to operate at 10× the speed of standard devices.

Here’s how it works. Remember that once a standard bus reset is given, all devices are waiting to respond. When a device is chosen via MATCH- ROM, all devices without that ID are essentially turned off (until the next bus reset).

Only one device can now reply to further commands, so it doesn’t matter what the timing is like, as long as it doesn’t create any timing that resembles a bus reset and as long as the master and slave agreed on the timing.

The reduced timing parameters for overdrive let communication increase by a factor of 10 to devices supporting the feature. When the bus reset is sent again (normal timing parameters), the overdrive device reverts to normal timing.

LAST STOP

Well, we’ve come to the end of this bus line. Although we could transfer to one of many alternative device discussions, there are too many to explore with only one token. I hope I’ve piqued your interest enough so that you research these 1-wire devices on your own.

Jeff Bachiochi (pronounced"BAH-key-AH-key") is an electrical engineer on Circuit Cellar INK’s engineering staff. His background includes product design and manufacturing. He may be reached at jeff.bachiochi@circuitcellar.com.