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LM75B: I2C error

JH Shin
Mastermind 7830 points
Part Number: LM75B
Other Parts Discussed in Thread: INA220

Hello,

My customer had a problem that only the I2C communication of the LM75B was not possible when the I2C line of the LM75B and the SFP module were connected together.

If they remove the SFP module there is no problem. When the SFP module is mounted, I2C communication with the SFP module is normal, but the I2C communication of the LM75B is not working.

This symptom only occurs on some but not all LM75Bs.

1. Is there a waiting time for normal I2C communication after powering on the LM75B? If yes, what is the waiting time?

2. The captured I2C waveforms of the LM75B are as follows.

  1) I2C start bit

  2) Normal

  3) Abnormal

Looking at the waveform above, there is no ACK response from the I2C of the LM75B when it is abnormal.

Please advise on the cause of the problem and how to solve it.

Thank you.

JH

  • 0
    TI__Mastermind 37965 points

    1. Power-on wait is not specified or expected. I would still suggest testing a delay as a troubleshooting step. There is a specification that LM75B should not be accessed more often than every 300ms.

    Can you share the schematic? If the address pins are not properly tied off, the LM75B could detect a different address, causing NACK.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Mastermind 7830 points

    Sorry for the confusion about the error symptoms.

    The customer re-reported the error symptom as below.

    1. While checking each device when the Linux OS boots, the above error that does not recognize the LM75B has occurred on some boards.

    2. The above error occurs only when the SFP module is mounted when the board power is turned off and the power is turned on for the first time after about 10 minutes of complete discharge.

    3. After Linux booting is completed, I2C communication with LM75B is normal.

    4. When the LM75B device that gives the above error at boot time is ported to a normal board, the same error occurs.

    Below is the customer's circuit.

    The INA220 is not mounted

    Q1. The datasheet only has 10kohm as the pull-up resistor for the I2C lines. Is there any problem using 4.7kohm?

    Q2. The requirements for the rising time of the LM75B VDD could not be found. Please let us know if you have any requirements for this.

    Q3. What can cause the LM75B to not send an ACK?

    Q4. Please let us know if there is any suspected cause for this symptom.

    Thank you.

    JH

  • 0 in reply to JH Shin
    TI__Mastermind 37965 points

    1. Pull-up is limited by 3mA in the Vol specification. Minimum pull-up for your situation is 3.3V/3mA = 1.1kOhm. 4.7Kohm is acceptable.

    2. No requirements for rising time, but a slow rise can make it difficult or impossible to meet other timing parameters. The rise time in your captures are reasonable for the clock frequency shown.

    3. R418, R419, R420 should be changed to zero ohm. There is very little leakage current to ground here, which can cause the state of the pin to be misinterpreted. These pins configure the address that the LM75B responds to.

    ren

  • 0 in reply to Ren Schackmann
    Mastermind 7830 points

    Hi Ren,

    Thanks for your reply.

    The customer has further question.

    The LM75B datasheet describes it as having a low-passed filter on the I2C line. Is there a noise specification that users must abide by for normal communication?

    If the user satisfies the VIN(1) and VIN(0) specifications of the Section 6.6 Digital DC Characteristics table, there will be no problem?

    Regards,

    JH

  • 0 in reply to Ren Schackmann
    Mastermind 7830 points

    Hi Ren,

    The customer monitored the VDD, SCL, and SDA lines of the LM75B immediately after powering on.

    1. Without SFP module

    2. With SFP module

    When the SFP module is mounted, the symptom that SCL and SDA are maintained at about 2.2V to 2.3V is observed. Could this symptom be the cause of the LM75B I2C error reported above?

    Thank you.

    JH

  • 0 in reply to JH Shin
    TI__Mastermind 37965 points

    There is no noise specification.

    I don't expect that the low voltage causes the problem, unless the I2C is trying to communicate during this period. I would prefer to see the first I2C transaction after power-on, and measure the time difference in the 2 scenarios.

    However, I expect that your problem is caused by your large address resistors. I previously suggested that you change these resistors to zero ohms.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Mastermind 7830 points

    Hi Ren,

    The customer tested by changing the pull-down resistance of the address pin to 0 ohm, but the symptom of no I2C response of the LM75B is the same for the first time when the power is turned on.

    Below is the I2C waveform capture image when there is no initial ACK.

    There is no problem with the quality of the I2C signal.

    Below are the captured images of the first power-on waveform and the second power-on waveform.

    The symptom of no I2C ACK occurs only when the left power is first turned on. When the second power is on from the right, it operates normally.

    Below is the captured power-on waveform after replacing the SFP with another manufacturer's.

    No first-time I2C errors occur.

    The customer suspect that the difference in waveforms at power-on causes the first I2C error.

    Please advise on the cause of this symptom.

    Thank you.

    JH

  • 0 in reply to JH Shin
    TI__Genius 9675 points

    Hi JH,

    Ren will be out this week so I will jump in to help support. Let me review the conversation and see if there are some additional debug steps that can be provided.

    Jalen

  • 0 in reply to Jalen Tate89
    Mastermind 7830 points

    Hi Jalen,

    Thanks for your support.

    The customer did further debugging and found that changing the power source did not cause this issue.

    Below is a comparison of the ramp-up slope between the original 3.3V input supply (yellow) and the modified 3.3V supply (red).

    Below is the ramp-up slope waveform of the changed power supply and the SDA and SCL signal lines.

    If the SDA and SCL signal levels stabilize before the 3.3V power supply stabilizes, as in the second power-on slope comparison in the previous inquiry, it is assumed that the no ACK issue does not occur.

    As explained earlier, the symptom without ACK occurs only once after the first power-on after mounting a specific SFP.

    1. Please tell us if there is any problem with the customer's LM75B circuit design.

       (FYI, changing the 4.7Kohm resistor on the address pin to 0ohm has been tested to not affect this issue.)

    2. Please tell us if the 3.3V, SDA and SCL ramp-up slope captured below could be causing the issue of no ACK being fired.

    Regards,

    JH

  • 0 in reply to JH Shin
    TI__Guru* 91146 points

    JH  - 

    what is the specific I2C address of the SFP being used when this issue arises? Jalen will surely take a look at this tomorrow as well. 

  • 0 in reply to Josh Wyatt
    Mastermind 7830 points

    Josh,

    The SPF module uses two I2C addresses, 0b1010 000x and 0b1010 001x.

    Both the SFP module causing the issue and the SFP module not causing the issue in the LM75B use the same I2C addresses.

    Thanks,

    JH

  • 0 in reply to JH Shin
    TI__Genius 9675 points

    Hi JH,

    It seems to me that the NACK is occurring when the SDA/SCL ramps slower than the Supply ramp.

    I would suggest lowering the pull up value closer to the minimum of 1.1kOhm, as Ren mentioned, to see if we can increase the rise time to stabilize before the supply voltage does.

    Jalen