My results so far show that the DS18S20 definitely does NOT know for sure if there is an external +5v supply. I had a network of 3 sensors on the desk, if I leave the +5v connection disconnected on all of them one reported that it was powered, the other 2 that they were not. They do all report Power = 0 if I connect the +5v to GND
On timing - Python loop reading 3 sensors, loop 100 times External Power cache(0) 15.3 seconds External Power cache(1) 3.8 seconds Self Powered cache(0) 21.7 seconds Self Powered cache(1) 5.3 seconds So the difference in speed for power seems consistent with your observations. Note - one of my sensors consistently returns power = 1 irrespective of what I do Question - what are the parameters for caching? From my results I assume that with cache(0) it forces a genuine read each time and with cache(1) it will return the latest value read and periodically refresh the cache. How often and can this refresh period be set? Thanks Mick On Sun, 2010-05-02 at 14:22 -0700, Marc MERLIN wrote: > On Sun, May 02, 2010 at 02:53:50PM -0400, Paul Alfille wrote: > > > As part of these investigations it seems to me that the parameter > > > 'power' does not actually mean that the device has external power, it > > > means that the ground lead is not connected to the VDD (+5v) lead. Can > > > anyone confirm this? > > > > Power comes straight from the chip's report. You can use either > > whatever you find experimentally, or what the datasheet says. > > Please post back what you find here, I'm now switching to a setup with a > poewred hub and some of my devices will be powered, some will not because > it's time consuming to get to them to add power after the fact. > > I did some quick test with DS18B20 in parasite vs powered mode. > I indeed found no way from the owfs interface to know if the device is > powered or not, but owfs must know since querying in parasite mode is not > the same code than querying in powered mode. > > In powered mode, an average of 10 uncached reads took 0.95s for 12 bits > In parasite, it took 1.35 sec for 12 bits. > > I expected the difference to be bigger but I guess not. > > But is owfs able to know which devices are powered and send requests in > parrallel to them and get the answer 1 sec later of all of them at once, > while doing parasite ones serially later, or does it just treat them all the > same? > > And just to confirm, it owfs does know if an individual device is > powered, does it not? > > Marc ------------------------------------------------------------------------------ _______________________________________________ Owfs-developers mailing list Owfs-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/owfs-developers
