• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 33 subscribers
  • Views 286 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TMP75: TMP75 unable to change resolution

Filip Carlsson
Prodigy 10 points

Part Number: TMP75
Other Parts Discussed in Thread: TMP1075

I have been trying to change the configured resolution from 12-bits to 9-bits.
According to the datasheet the resolution of the shelf should already be 9-bits, but after trying to read from the sensor I got 12-bit resolution even without any prior configuration.

I am writing 0x00 to the configuration register 0x01 after which I set the pointer register to 0x00.
After this I am reading the temperature register into 2 bytes but I consitently see bits 4-7 with value 1.

Just to check wether my configuration parameter was working I changed the polarity of Alert pin using configuration command 0x04 and connected an LED to te pin. This resulted in the LED lighting up.

Any ideas on why I am still seeing 12-bit resolution? 

  • +1
    TI__Mastermind 45255 points

    Hi Filip,

    On the original TMP75, the ADC needed more time to provide the extra bits. The ADC ran continuously, and the Resolution setting was a way to trade off accuracy with update rate. That is to say, a new temperature result would be available every 27.5ms (in 9-bit mode) when the ADC conversion completed. If you wanted 12-bit data, you would only get a new result every 220ms. The power consumption was the same, because the ADC never stopped in any mode.

    On a modern design, like TMP1075, the ADC is both faster and higher resolution. The TMP1075 makes its 12-bit measurement in 5.5ms where the original TMP75 needed 27.5ms to make only a 9-bit measurement. It's for this reason that the TMP1075 does not change it's resolution. Instead, in order to make the TMP1075 compatible with TMP75's update rate, the TMP1075 goes to sleep during the remaining ADC conversion time. This provides power savings over the old design.

    Over the years, TI has had to make changes to manufacturing flows. It's for this reason that your TMP75 does not have the original chip design inside it. Your TMP75 will always report 12-bit data, because there is no benefit to eliminating those extra bits. The format of the 9-bit and 12-bit data is 100% compatible, as well. If you don't want the extra bits, you can choose to remove them during your software processing of the result.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 10 points

    Thank you  that makes sense!

    Now I can stop wondering why I saw the weird results that I saw and just accept that that's how it is supposed to be.