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Original question:

P82B715: Verification on P82B715DR design

P82B715: Why pull up at 3.3V but output shows 4.2V?

Poh Kok Keng
Genius 5512 points
Part Number: P82B715
Other Parts Discussed in Thread: TCA9517

hi,

i did I2C pull up @3.3V on pin 3 and 6. VCC is 5V. however, when i did a measurement on pin3 and 6 (the I2C on Sx/Sy (after U25), it shows ~4.2V and not the expected 3.3V. 

Can you explain how to make the i2C output at 3.3V when HIGH? 

BR,

Felix

  • 0
    Guru 317250 points

    The two sides are not isolated from each other, i.e., this device cannot be used for level shifting.

  • 0
    TI__Mastermind 47978 points

    Hi Felix,

    It looks like you have two different voltage levels on either side of the P82B715. Like Clemen's stated, this device doesn't support level translation, meaning you cannot have expected behavior if you are operating at two different voltage levels.

    Looking at your design, it looks like the pullup on the 3.3V rail is slightly stronger than the pullup on 5V side (1.5kohm vs. 2.2kom). This means that the voltage on SDA/SCL will favor the 3.3V logic side slightly. Looks like you are measuring 4.2, which is about halfway between 5V and 3.3V. A possible solution would be to make the pullup resistor on the 3.3V side stronger (<1.5kohm), and make the 5V pullup weaker (>2.2kohm). When doing this, you must keep in mind the min/max pullup resistance values and make sure you can reach a Vol as well as the rise time margins put on by the I2C standard.

    Otherwise you would need to look into a I2C bus repeater & level shifter. A device such as the TCA9517 could solve your problem. Unfortunately, a lot of these types of devices are out of stock due to the semiconductor shortage. 

    Regards,

    Tyler