Problem with my 1-wire found

Last week I posted about an issue with my 1-wire temperature sensors.  I believe I have found the issue.

I am using DS18B20 Waterproof Temperature Sensor assemblies that I picked up on amazon.com.  I looked in a datasheet for a DS18B20 from Maxim Integrated and found a note at the bottom of table 1.

DS18B20_table1

“The power-on reset value of the temperature register is +85°C” means to me that when my database is reporting “85” that means at the time of the temperature reading, the sensor was in its power-on reset state.  I also note later in the document that the “Minimum Supply Voltage” is listed at “+3.0 volts.”  Since my pi is powering the DS18B20 sensors from a 3.3V supply pin, it is possible that this is being pulled too low (below 3.0V) and causing the sensor to be reset.

I was also reminded that every “Transaction Sequence” (communication) on the 1-wire system begins with an initialization step that includes the master (raspberry pi) transmitting a reset pulse on the 1-wire bus data line. This is then followed by a presence pulse from the slave(s) on the bus.  This timing is pretty specific, and if incorrect the data won’t be correct.  It is possible that there being many devices on the bus or extra long cabling between master and slave(s) on the bus could cause this timing to be incorrect.  This would be helped by a stronger pull-up resistor on the data line, meaning less resistance.  My piHouse uses a 4.7K ohm resistor for this pull-up, following the datasheet recommendations.  And with only 4 slaves and cable run of a couple-hundred feet at most, I don’t think this will be necessary to change.  Plus, since I hot glued the thing in place, it might be difficult to change the 4.7K resistor.

1-wire

Since the maximum operating voltage listed for the DS18B20 is shown to be +5.5V, I will change the V+ source for the temperature sensors to one of the +5V outputs on the pi and see if this makes a difference.  If not, I will endeavor on changing the 4.7K to something smaller.

Unfortunately until then, my graphs will keep looking like this

show_graph_2014-10-28.cgi_

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