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Mike_GTN
07-01-2009, 09:22 PM
·Hi All,

·I have a project that is in the planning stage, but will require the use of some remote mounted Dallas DS1621 Thermometers.
·These will be some approx 10 metres away from the Propeller based controller. So obviously I need to extend the bus range of my
·I2C connection.

·On a web search the major part that was·returned·is the Philips/Texas P82B715·device, has anyone here used this device with good
·results? Or should I be looking towards a different/device solution?

·The Propeller will have attached locally an addiional 24LC256 EEprom and DS1307 RTC.··I propose to power the DS1621's in the remote
·location with a single dedicated power supply.·

·Thank-you for any help offered.

·Regards

·Mike.

Leon
07-01-2009, 09:55 PM
The data sheet mentions up to 50m, so you should be OK.

Leon

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Mike Green
07-01-2009, 10:03 PM
I haven't used the P82B715, but, from the datasheet, it looks like a good part for the job.

Have you considered using a 1-Wire thermometer like the Dallas DS18B20-PAR? This can be powered from the Propeller. It would require two I/O pins, one for the 1-Wire bus and one to control a "strong pullup" MOSFET. There's code in the Object Exchange for 1-Wire use. I'm not sure if it provides for the MOSFET, but that could be easily added since only certain 1-Wire operations require the additional current supplied.

The 1-Wire bus is designed for use with many sensors scattered over a significant distance (thermostats in a building).

pjv
07-02-2009, 01:38 AM
Hi All;

Actually to go further on what Mike was saying, the "strong pull up" requirement is while the sensor is doing a conversion or when it is writing to its internal flash. At those times there is no communication with the sensor, hence the port bit can be turned into an output and driven high for that period and effect the "strong pull up". For safety, I stick a 100 ohm resistor in series with the port bit. The standard 4k7 pull up resistor to Vcc is still required of course as the pull up for normal communication. In this manner only a single I/O pin is required..... the second port bit and the mosfet can be scrapped.

Cheers,

Peter (pjv)

BradC
07-02-2009, 08:32 AM
I've used these (DS18[SB]20) in some environments very close to their rated limits. When getting them very hot, parasite power can be a problem as the silicon leaks quite badly. I've had best results using them on +5v with three propeller pins. One Mosfet to ground to perform a pull down. One with 10k series resistor to sample the data line and one to a PNP transistor for strong pull up. This has provided very good results near the temperature extremes over long lines with multiple sensors.

If you are going to exceed about 70Deg C you really need to look at a separate +5v rail though.

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