• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 33 subscribers
  • Views 693 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.

TUSB8020B: I2C to flash pull up resistor value bad

John Swenson76
Intellectual 320 points
Part Number: TUSB8020B

We are using the TUSB8020B with a flash chip. We had problems with getting the programming correct but finally worked it out and did a production run of the boards. While flashing these we found that about 40% would not flash.

It turns out the voltage on the SCL/SDA lines were 1.5 to 1.8V which is a pretty big drop across the 10K resistors. I did some tests and it looks like the hub is pulling some current from these lines which is causing the large voltage drop. It seems the much lower than expected upper voltage is hitting the receive threshold in either the hub or flash chip. Just normal manufacturing tolerances seem to be causing the 40% failure.

Changing the pull up resistors to 4.7K changes the upper voltage to 2.5V which seems to work just fine. It seems the protos we did just happened to be in the 60% that work with 10K resistors. (BTW I'm using 0.5% resistors so it should not be a resistor tolerance issue)

The documentation should probably be changed to recommend a different value.

Thanks

John S.

  • 0
    TI__Guru 60870 points
    Hi John,

    Thanks for the feedback, can you point me to the documentation that has the 10K resistors on SDA/SCL? You are correct that 10K is not a strong enough pullup. We typically recommend 4.7K or smaller to override the internal pu/pd resistors and usually recommend 1K/2K pullups on SDA/SCL. That document needs to be updated.

    Thanks!
    JMMN
  • 0 in reply to JMMN
    Intellectual 320 points
    I checked the documentation and it never says what value to use. I remember that what I did was look up the documentation for another part I had used that uses an I2C interface and it said to use 10K.

    The section on the EEPROM in the spec sheet just says to pull them up. In the application section the schematics never show the SCL/SDL pins hooked up to anything.

    So the docs don't give the wrong value, they don't specify any value.

    Thanks,

    John S.
  • 0 in reply to John Swenson76
    TI__Guru 60870 points
    It looks like the other hubs had actual values in the EVM guides, but this one didn't. We'll get that added.

    Thanks again,
    JMMN