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[nickNetduino]

I've been able to natively read the temp from a single DS18B20 on my new Netduino Plus 2 using non-parasite power. My trouble comes with trying to read multiple DS18B20's using parasite power. I can see the devices ok (populated into "deviceList") however I am getting erroneous readings. I'm sure my writebyte's/readbyte's must be off or something. Here is the pertinent code and a link to the datasheet (using page 18, example 1 as a reference). Please let me know if you have any insight. Thank you so much!

http://datasheets.ma.../ds/DS18B20.pdf

static float getOWTempByAddress(OneWire owBus)
        {
            ArrayList deviceList = owBus.FindAllDevices();

            foreach (byte[] device in deviceList)
            {
                Debug.Print("Found ID: " + bytesToHexString(device, true));
            }

            byte[] deviceToCheck = (byte[])deviceList[0]; //lets just look at the first device as a test

            float TemperatureC = 0;
            int tempLow = 0;
            int tempHigh = 0;
            int temptemp;

            owBus.TouchReset();
            owBus.WriteByte(0x55); //match rom
            foreach (byte theByte in deviceToCheck)
            {
                  owBus.WriteByte(theByte);
            }
            owBus.WriteByte(0x44); //start temp conversion
            while (owBus.ReadByte() == 0) ; //wait while busy
            owBus.TouchReset();
            owBus.WriteByte(0x55); //match rom
            foreach (byte theByte in deviceToCheck)
            {
                  owBus.WriteByte(theByte);
            }
            owBus.WriteByte(0xBE); //read scratchpad
            tempLow = owBus.ReadByte(); //LSB
            tempHigh = owBus.ReadByte(); //MSB
            temptemp = (((int)(tempHigh) << 8) | tempLow);
            TemperatureC = temptemp * 0.0625f;

       
            }
            return (float)((TemperatureC * 9.00 / 5.00) + 32.00);
        }

Edited by Chris Walker, 11 November 2012 - 05:19 PM.
added [code][/code] tags

[CW2]

I think the problem is that temperature conversion (when using parasite power) requires strong pull-up on the data line, i.e. step 6 "DQ line held high by strong pullup". You'd probably need to use the external MOSFET, as shown on Figure 4, page 6 in the datasheet.

Also, you'd need to issue the complete read sequence for each sensor (Reset, MatchROM, ReadScratchpad), because only the device that exactly matches the 64-bit ROM code sequence will respond to the function command issued by the master; all other slaves on the bus will wait for a reset pulse. The conversion can be started simultaneously (Reset, SkipRom, ConvertT), but the read-out requires the complete sequence for each sensor.