TMP75: Reading the same temperature repeatedly.

Fabian Wärn

Part Number: TMP75
Other Parts Discussed in Thread: TMP1075

Switching from TMP1075 to TMP75. Configured device to shutdown mode, alert pin in active low and comparator mode (although not used), 1 fault and 220ms conversion rate. This way we can run the one-shot conversion rather than continuous mode. The communication looks fine at both but for some reason the TMP75 temperature reading always returns the same value. The module is connected as 8.2 TypicalApplication chapter suggests in datasheet. Supply voltage is 3.3V. The configuration has also been done to run TMP75 in continuous mode without success. This configuration also works fine on TMP1075.

I2C communication is set to 50kHz and looks like below on TMP75.

Picture 1 is setting configuration and reading it to confirm set. Picture 2 is setting one-shot. Picture 3 is reading the temperature.

pic1

pic2

pic3

over 3 years ago

0 Jesse Baker over 3 years ago

TI__Expert 7950 points

Hi Fabian,

Thank you for posting to the sensing forum.

It looks like you are setting the configuration register properly for operation in one-shot mode. I have a few questions to help with debug:

1. You're sending a one-shot command (writing 1 to the OS bit) like you did in picture 2 each time before you want to read a temperature, right?

2. Are you waiting for the temperature conversion to complete (220 ms) before trying to read the temperature? It's possible you may be reading the temperature register before the temperature conversion is complete.

3. How many consecutive identical temperature readings are you seeing? The device is reading 0x1750 (23.3125) each time?

4. How are you varying the temperature of the device?

Best regards,

Jesse

0 Fabian Wärn over 3 years ago in reply to Jesse Baker

Prodigy 20 points

Yes precisley. the one-shot is done then wait 1 whole second before i read temp data then wait 1 sec before one-shot again and repeat this pattern. So should have time to do conversion.

Yes the device is reading the same temperature over again for at least several hours. I try to vary the temperature by putting my finger on the top of the chip.

For some reason when i disconnect the chip from all power sources and let it be for at least 1h the chip sometimes (not always) give a new temperature. Is there a possibility the chip is broken?

+1 Jesse Baker over 3 years ago in reply to Fabian Wärn

TI__Expert 7950 points

Hi Fabian,

Thank you for the additional information.

1. Can you run the TMP75 in Continuous Conversion mode and share a capture of the I2C traffic (several temperature reads) before, during and after heating up the device?

2. Can you measure the current draw of the device after sending a one-shot? There should be a short spike in current when the temperature conversion happens.

Best regards,

Jesse

0 Fabian Wärn over 3 years ago in reply to Jesse Baker

Prodigy 20 points

I can share the communication now but sadly the measurement of current have to wait until next week as I am missing the equipment. But i will return with this info.

In continuous conversion the communication looks like this. (Today the temp has updated with a very high temp value).

0 Jesse Baker over 3 years ago in reply to Fabian Wärn

TI__Expert 7950 points

Hi Fabian,

Could you share your schematic as well?

In the continuous conversion communication shown above, did the device continue to repeatedly output the high temperature value?

Best regards,

Jesse

0 Fabian Wärn over 3 years ago in reply to Jesse Baker

Prodigy 20 points

Hi Jesse! Based on previous answer about checking the current draw at one-shot it is determined that the sensor is faulty, draws to little current, and makes no temp conversion. But to answer your last questions.

The schematic is identical to the typical example in datasheet 8.2.
In continuous conversion it repeatedly output same high temp value.

Thanks for your time and help!

// Fabian

0 Jesse Baker over 3 years ago in reply to Fabian Wärn

TI__Expert 7950 points

Hey Fabian,

I just tested a TMP75 in one-shot mode and was able to get different temperature measurements (see I2C communication below). Were you able to test your setup with a different TMP75?

1. Set Configuration Register