Am 02.10.2015 um 22:25 schrieb Loren Amelang:
>
> So is your 1K pull-up really the specification for a 3.3V supply?
> Could it somehow change my temperature readings?
>
Loren, the bus pullup resistor only affects how fast the Low->High edge
rises after all devices on the bus disable their output transistor.
The actual rise time is governed not by the resistor alone but by the
resistor and the cable capacitance and the capacitance of the parasite
power circuits inside the slave chips. So there's no exact value for
that resistor.
A general rule for bus pullups (1W, I²C etc.) is to make them as low
as possible without overcurrenting the output transistors. That way the
rise time is as short as possible and the cable capacitance can be much
higher without an impact on the bus timing.
The onewire datasheets say the pullup resistor value should be between
1.5kOhm and 5kOhm, which means the current through the resistor (and the
output transistor) should not exceed 3.3mA. So it follows the minimum
value for that resistor at 3.3V is 1kOhm.
Always use these 1.5kOhm at 5V or 1kOhm at 3.3V.
The only exception is when you have a battery-powered device which has
to save power. But in that case I would make the pullup switchable (use
another output pin for it) and use the bus only once in a second or so.
That way you save both power and have a reliable bus.
> Now I get solid, repeatable readings, but one sensor reads 2 degrees
> C above what it read in another system with a 5V bus, and the other
> reads 5C high. (I got distracted by other projects and never
> resolved how that could happen...)
>
If the sensor tells it has a different temperature, it actually has a
different temperature. These are calibrated devices so their readout can
be trusted.
However, your outer setup may actually heat up the sensor. Without
knowing the actual circuit and mechanical setup, it's hard to tell what
may be the culprit.
Kind regards
Jan
------------------------------------------------------------------------------
_______________________________________________
Owfs-developers mailing list
Owfs-developers@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/owfs-developers
