[COLUG] OT: OneWire hardware hacking

[Josh]

Jim wrote about inductance and other interesting EE aspects of my 
question.

Actually, the 1-Wire interface takes care of all of those problems for 
you.  The chips include things like small bits of memory, SHA-1, A/D, 
switches, and environmental (temperature, humidity).  You can literally 
have a long twisted pair (up to 1km I think), one line being ground and 
one line being data, and string a bunch of them in parallel.  The chips 
are powered by capacitors that are charged when the data line is high. 
For increased reliability you can have a three-line "pair" and supply +5 
volts.

On the host side, there are things like TINI (a self-contained Java-based 
computer with ethernet), Serial, and USB host adapters (in complete form 
or chipsets); Maxim/Dallas offers Java and C libraries for free to talk to 
these things.

Mike Schoenborn asked about the timing of the device in question.  It is a 
DS1821, with three leads (or three connected leads in the case of SOIC): 
Ground, +Vcc, Data.  This device is not parasitic so it needs +Vcc at all 
times.  In normal mode, the Data line is a 1-Wire interface used to get 
the temperature or set the registers of the device.  If you give it the 
special 16-pulse signal, it changes to thermostat mode where the Data pin 
will go high or low depending on the current temperature of the device and 
the trip points set in its registers.

My timing was off before:

The Pulse "low" time can range from 0.1-10 Microseconds
The pulse "high" time seems to be able to range from 0.1 microseconds to 
infinity
The pulse transition time must be less than 100 nanoseconds

Pages 16 and 17 of this datasheet provide more details:

http://datasheets.maxim-ic.com/en/ds/DS1821.pdf

Cheers,
-J