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TCA9511A: Backplane application

Art Mecina
Genius 16585 points
Part Number:

Hello Guys,

Good day.

Our customer have a question regarding TCA9511A. He knows that the datasheet says that the part is recommended to be on the peripheral card as opposed to the backplane. However, he have an application where he need to place it on the backplane. Is this feasible and what are the ramifications?

He is also interested in general on how the pre-charge circuitry may interact with the main I2C bus if a part like TCA9511A with pre-charge is placed on the backplane.

Thanks in advance!

Art

  • 0
    Guru 271360 points

    Section 10.2 of the datasheet says:

    The typical application is to place the TCA9511A on the card that is being inserted or connected to a live bus, rather than being placed on the live bus. The reason for this is to provide maximum benefit by ensuring that the bus stays disconnected until an idle condition or stop condition is seen.

    Section 10.3.1 says:

    There are a few considerations when using these hot swap buffers. It is NOT recommended to place the TCA9511A on the backplane connector as it cannot isolate the cards from one another which will possibly result in disturbing on-going I²C transactions. Instead, place the TCA9511A on the I/O peripheral card to maximize benefit.

    The stop/idle detection runs at startup, i.e., when VCC and EN go high. To get the benefit of that when the TCA9511A is on the backplane (where it is always powered), you have to keep EN low normally and force it high only after a card has been inserted.

    The pre-charge circuitry goes through 100 kΩ resistors, so it will not noticeably affect the active I²C bus.

  • 0
    TI__Guru 56660 points

    Art Mecina said:

    Part Number: TCA9511A

    Hello Guys,

    Good day.

    Our customer have a question regarding TCA9511A. He knows that the datasheet says that the part is recommended to be on the peripheral card as opposed to the backplane. However, he have an application where he need to place it on the backplane. Is this feasible and what are the ramifications?

    [Bobby] You would essentially lose the precharge feature of the device depending on how you set up the schematic/system. If a card were removed, you would either float the connector side (assuming the card has the pull up resistors on it while the backplane connector didn't). At this point, noise could couple onto the connector side and cause glitches on the upstream side (again assuming your connector side doesn't have pull ups and you were relying on the pull ups of the card). Now if we assume that the connector side did have pull ups, then you would end up losing the hotswap feature entirely since all the 'smart' checks were completed at power up and the two sides were connected due to the pull ups.

    The precharge feature of the device only works upon power up since the design for this device assumed the device would sit on a daughter card and would only be momentarily triggered upon insertion of the daugther card into a backplane.

    He is also interested in general on how the pre-charge circuitry may interact with the main I2C bus if a part like TCA9511A with pre-charge is placed on the backplane.

    [Bobby]
    You would need to not populate pull up resistors on the connector side to see the 1V precharge. If the Pull up resistors were present, then the 1V precharge would just turn off upon power up since you would yank the SDA/SCL line up to Vcc (100k pull down to 1V is weak compared to a ~4.7k or 10k pull up to Vcc).

    Thanks in advance!

    Art

    For hot insertion protection on the backplane, using an I2C switch would be better.

     

    You may want to give this app note a read as well. It discusses the TCA9511A in more detail than the datasheet for hot insertion and its internal checks. Also some discussion as to how to design a backplane for hot insertion.
    http://www.ti.com/lit/pdf/scpa058