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TCA9548A-Q1: I2C low voltage level issue in TCA9548ARGERQ1

Sampath Burra
Prodigy 10 points

Part Number: TCA9548A-Q1

TCA9548ARGERQ1

Hi, 

I have been using TCA9548ARGERQ1 I2C Switch in couple of our projects. I need help for two issues facing in I2C which are mentioned below,

i) I found that even though no load is connected to TCA9548ARGERQ1 I2C Switch. For I2C SCL & SDA, VOL (Low Voltage Level) not maintained at 0V. 

   I have checked with different pullup resistor values (1k, 1.5k, 4.7K & 10K) but could not be able to reduce the VOL value to "0", it is maintained at 200mV for 4.7K Pullup. Please find the reference image below.

     

      I could not be able to find original reference images in TCA9548ARGERQ1 datasheet.

      It is in the range of recommended working voltage of the device (Max. VOL value supported is 0.3*1.8 = 0.54V), but we want to reduce that voltage to "0V"? 

     Kindly let me know why it is happening like this and recommendations to avoid such issues.

ii) One more issue is when connected with the load, there is toggle in low voltage level as shown in figure below. Device is working fine in below situation too.

    But I need to solve this issue. In this case also we checked the issue with different resistor values like 33Ohms (not working), 1K, 1.5K, 4.7K, 10K and 105K. In all above cases same behavior is maintained.

    Please help me with this issue.

    

Thanks,One more issueii22fc

Sampath Burra.

  • 0
    TI__Mastermind 35648 points

    Hey Sampath,

    It's not uncommon to not get all the way to 0V on the VOL since you'll have some resistance to ground on the FET when pulled low. But it is interesting that changing the value doesn't change the value. Just to confirm you're saying that the pull-up values don't impact the VOL at all? I would expect that they would change it a bit but never really get down to 0V.

    On your second point, it does look like something pulls the line strongly to ground momentarily before being released. It could be a timing thing with regards to how the I2C signal is reacting with something else in your system. 
    Do you have a schematic you can share so I can see what could possibly causing this issue? 

    Thanks,
    Rami

  • 0
    Guru 322000 points

    1. The pull-up resistors are the load. With a switch, the pull-ups of all enabled channels act in parallel. It is not possible to get down to 0 V. You can try to get near zero with higher resistor values (on all bus segments), but this might also require reducing the I²C speed.

    2. Any device can pull an I²C bus line low; you get different voltage levels if the devices have different drive strengths, or of multiple devices happen to pull down at the same time.