DS1621 Digital Thermometer and Thermostat

Ahmad Zaki Yamani is a student of Politeknik Negeri Bali (PNB). He came to my house and asked me about how to use DS1621, a temperature sensor chip made by Dallas Semiconductor. He bought it in Surabaya City. Well, I have never been using it before, and I said, “I don’t know how to use it”. He told me that he brought the data sheet of the chip. I read it carefully. It seems that he got it from http://www.maxim-ic.com. For me, it is difficult to understand how does the chip work to convert the temperature to the logic form. But the data sheet describes clearly about data communication between this chip and its controller using 2-wire serial bus. The data sheet informs about thermostat feature with thermal output alarm (Tout) pin. This output is active when the temperature exceeds a user-defined temperature (TH). It sounds interesting for me and I agree to help him to use the chip as digital thermostat with PIC 16F877 micro controller.

After a few days of experiments, we found the right way to make it work as a digital thermostat. Here are the algorithm :

1. Set the High Temperature (TH) Register.

2. Set the Low Temperature (TL) Register.

3. Set the Configuration Register with 03 Hex (Active High Polarity and 1-SHOT mode).

4. Start temperature conversion by sending START CONVERT command .

5. Give it a delay about 1 second.

6. Check bit 6 of Configuration Register. If it is high, the temperature exceeds high temperature limit.

7. Check bit 5 of Configuration Register. If it is high, the temperature belows low temperature limit.

7. Give it a delay about 1 second.

8. Stop temperature conversion by sending STOP CONVERT command.

9. Apply a delay as needed and loop back to step 3.

By checking bit 6 and 5 of Configuration Register, one able to know when the temperature exceeds or belows temperature limit. Dallas Semiconductor writes that, “Tout becomes active when temperature meets  or exceeds the value stored in TH register and will stay active until the  temperature falls below the temperature stored in the  TL register.” For us, Tout doesn’t work as described by the producer.  We probe Tout pin using logic probe. It gives low logic when the temperature belows TL register but it doesn’t give high logic when the temperature exceeds TH register. It gives only 0.8 volt output. When we decrease the temperature belows TL register, Tout still doesn’t give low logic.

Does anybody know how to use Tout pin of DS1621? I am waiting for your comment or advise. Thank’s.

By Sapteka. http://www.sapteka.net