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TMP275: seeing blip in sda data at clock 8

J.P.
Intellectual 300 points
Part Number: TMP275

Good afternoon,

I probed the SDA and SCL (triggered on scl) bits using good probing techniques, I noticed that there is a "blip" at the 8th clock bit with a consistent shape. we are getting valid data but I am not sure what this is or why? I verified that this is the case on 2 different temp sensors

temp sensor 1

temp sensor 2

  • 0
    TI__Guru**** 244380 points
    Hello JP,

    Can you please zoom in on the 8th clock to show what blip is being referred to. On the current images we only see regular clock pulse, so not sure if we are on the same page.
  • 0
    Intellectual 300 points
    Good afternoon,
    The blip is on the sda line. I have attached a 3rd sensor that is on a completely separate board that shows a similar thing
     
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    Julia Pflibsen
    Hardware Engineer
     
    1 720 890 4528 Direct
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  • 0 in reply to J.P.
    TI__Guru**** 244380 points

    Hello Julia,

    J.P. said:
    I noticed that there is a "blip" at the 8th clock bit with a consistent

    OK, so that got me looking at the clock line. Anyways, the blip is after the 8th clock which is when the slave sends an ACK. For the slave ACK (which is active low), the master must release the SDA line. As a result the pull up takes effect which causes the SDA to go up. But at the same time the Slave starts driving the SDA line. As long as the SDA line changes when the SCL is low, it should not be an issue.

    As per the I2C specification, the SDA is sampled after the rising edge of SCL. So what you see is perfectly normal.

  • 0 in reply to J.P.
    TI__Mastermind 35795 points

    Hi Julia,

    Most likely you have a series resistor on your SDA line, and the blip is changing polarity based on which side of the resistor you monitor.

    As long as logic levels (Vih and Vil) are met, the blip should cause no issue.

    thanks,

    Ren