On Thu, Mar 3, 2011 at 9:20 AM, Denny Esterline <desterline@gmail.com> wrot=
e:
> Ok, skipping all the off-topic stuff.
> Maxim/Dallas's one wire parts use the phrase "strong pullup" to
> indicate the line SHOULD BE DRIVEN HIGH. The pullup resistor is not
> enough, a PIC's weak pullups is not enough, the combination is not
> enough. The specs vary across the product range, but holding 90% of
> Vcc at 10mA is a good place to start.
> This is only necessary for the parasite powered parts, and only during
> certain times (like doing a conversion or writing to EEPROM).
>
> Some of the data sheets show an external FET controlled from another
> output pin to do this, many people have successfully toggled the TRIS
> bit and used the PIC to drive it high, including myself.
>
>
> It's been a while since I had my head in that code, but as I recall
> there's a certain time limit. Strong pullup _must_ be active within a
> certain time of issuing the command. It bit me on the backside because
> I had about 10 sensors to watch and the software had several nested
> calls to manage them all. In the end I had to separate out a couple of
> the routines to meet the timing requirement.

Thanks for the clarification.
--=20
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