1-Wire Signalling

1-Wire is a signalling protocol developed by Dallas Semi that has been around for over 10 years.  It’s actually a two-wire physical connection, but one of the wires is GND.  1-wire masters can power small devices and communicate with them over these two wires.

1-wire USB bus master
1-wire USB bus master

I learned a little bit about 1-wire signalling recently.  It uses an open-drain pull-down driver with a pull-up termination resistor.  To signal a “1”, a device pulls the bus down for a very short duration – from 1 to 15 uS.  To signal a “0”, a device pulls the bus down for 60uS.  A very long pulse – 480uS – resets everyone on the bus.

Nowadays, bus masters implement software control of network parameters.  Such things as slew rate and timing are adjusted dynamically by adaptive algorithms in the bus master.  I was aware of adaptation in very high-speed signalling systems for memory, ethernet and IO, but had not really encountered it’s use in a lower-rate system.  Here, adaptation helps with reliability and distance rather than speed.