Dallas IButton 1990 Presence pulse

[enigma]

Hi All

Has anyone worked out how to retrieve the presence pulse for an IButton (or any other simple method) to determine if a iButton is actually there before you try to grab any other info from it?.

Cheers Pete

[manwolf]

Take a look at this.

http://datasheets.maxim-ic.com/en/ds/DS1921G.pdf


Presence detector acknowledges when reader first applies voltage.

[Darrel Taylor]

I'm not sure why this isn't in the manual. Maybe they just forgot it was there. :)

You can have an optional Label after the OneWire commands.
If the Presence pulse isn't received after the reset, it will jump to that label.

This line will simply detect presence or not without trying to send data.
OWOUT DQ, 1, [], NotThere ; Detect if OW device is present
...
NotThere:

ADDED: Or, since you are using a ROM only device, you can do the same thing with OWIN. Saves a little code space.
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[manwolf]

Found this very helpful. even has code.

http://www.maxim-ic.com/appnotes.cfm/appnote_number/159

[enigma]

Hi All

Thanks, the data sheet is useful and thanks also for the code Darrel. The data on the command is a little sparce, I could see that you could force a reset but couldnt find any way to trap it with a timeout.

Cheers Pete