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TMP75 / Applying I2C Compatible Temperature Sensors in Systems with Slow Clock Edges

Other Parts Discussed in Thread: TMP75, LM75A, TMP75C, LM77

Hi all

LM75 series have the following problem, my customer apprehend this problem for the application.
・Applying I2C Compatible Temperature Sensors in Systems with Slow Clock Edges

So I would like to recommend TMP75 series.
I couldn't fine same application sheet in TMP75 web site.

Q1 : Does this device have the I2C problem?

Q2 : What is the difference point TMP75 series and LM75 series? 


Best Regards,
Sho Ogane 


  • Hi Sho,

    Could you provide a specific problem? Maybe I can help. In the past, I usually recommended customer using the latest released of LM75A. Here is a link that I discussed when customer faced a I2C serial interface issue.

    e2e.ti.com/.../159833

    I will send you later the comparison between TMP75 and LM75 series.

    Aaron
  • Hi Aaron san

    Thank you for your reply.

    A specific problem is the following.
    ・The master in this example has a very slow fall time on the falling edge of its SCL and SDA signals, on the order of the 300 ns maximum allowed in the specification. The slave ACK bit, however, has a very fast falling edge, on the order of 5 ns, without a 300 ns delay.  
    This problem causes protocol error because of SDA's fast fall time. 
    So the SDA signal has to wait 300ns from SCL's falling edge, my customer asks us whether TMP75 series have this I2C rule or not.

    Thank you, I am waiting your answer


     Best Regards,
    Sho Ogane

  • Hi Aaron san

    Sorry, I could't access the following page.
    e2e.ti.com/.../159833

    Please tell me the URL again.

    Best Regards,
    Sho Ogane

  • Hi Sho-san,

    What is the pullup resistor value on SCL and SDA? Do you know your stray capacitance?

    Aaron
  • Hi Aaron san

    Sorry, I don't have their information.
    Why do you need their information?

    Have you see the following URL?
    http://www.ti.com/lit/an/snoa558a/snoa558a.pdf

    My customer is using LM75A, but he located this URL and would like to change the temperature sensor from LM75A.
    So I recommended TMP75C.

    Is this URL application note basic I2C standard?
    Is this URL application note TI's requirement between MCU and TI's device(LM75A/B/C)?

    Can I recommend TMP75 series as improved device(URL issue) to my customer?

    Best Regards,
    Sho Ogane

  • Hi Sho-san,

    The SDA line signal changes during the falling edge of the SCL. I suspected that there might be a strong pull-up that caused the master generates a very slow falling edge, where it placed the device into a fault of START or STOP condition. The edge of the SDA and SCL are probably falling at the same time created a zero hold time, and SDA falls below VIL before SCL making it looks like a START condition or the other way of STOP condition. I would need your help if you can ask customer:

    1. What is the value of the pull-up resistor?

    2. Can you provide a scope photo when this happens?

    Aaron

  • Hi Aaron san

    I am sorry for the late response.

    I hear about the value of pull up resistor, so please wait.

    I attached PDF about I2C CLK and SDA wave of LM77.
    Please check the PDF file and give me your comment.

    My customer would like to know where this device can follow a rule in atiming of I2C.
    And how about TMP75?

    Best Regards,
    Sho Ogane

     I2C Bus(LM77).pdf

  • Hi Aaron san

    I got pull up resistance information.
    The value is 2.2kΩ.

    Best Regards,
    Sho Ogane

  • Hi Sho-san,

    The value 2.2kΩ is typical value that shouldn't be an issue. By the way, could you please give the raw file of your scope photo? It is much easier for me to zoom in.

    Aaron