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TCA9406 ISSUE

david c bassett69540
Intellectual 425 points
Other Parts Discussed in Thread: TCA9406

Hi ,

i can't believe i actually am having an issue with an 8 pin device, but here is my problem: nothing but a few glitches as an output.  my thought is there must have been an enable/power race problem, and i need to toggle the enable to check it out.  looking for some insight here.

here is the setup: stellaris LM3651 on a standard 3.3V side, talking fine to an i2c port - commands and responses are fine. so, SDA and SCL with 3.3V pullups on them and 3.3V is wired on the B side (higher voltage side).  on the other side, i have SDA and SCL pulled up to 2.8V going in to nothing but a beagle protocol analyzer and also a logic analyzer. Vcca and OE are tied to the 2.8V rail.

here is the behavior: if the analyzers are moved to the 3.3V side, both see clock and data fine and decode command and responses fine.  when they are moved to the 2.8V side, i see SDA and SCL both pulled high, and several down 8nS "glitches" and that is about it. actually, a clock glitch, then 3 clock and data glitches 42uS later, and then silence on the bus.

i have tried this on two separate devices with the same effect.  even though the app note says you can tie OE with Vcca, i have a feeling the slicers inside the part might be acting to cut off the output. my next effort is to wire the OE to the 3.3V side GPIO and pull that down and then high again to see if that works.

 

any ideas here?

  • 0
    Intellectual 425 points

    Hi again,

    here is some updated information:

    i had wired OE through a 10K pulldown and wrote stellaris code to pull it high, then low and then high again.  i checked to make sure the pin was doing that if left unconnected, which it did.  but, wired through both a 10K and 5K pulldown, the OE never left 3.3V even when not connected to any pullup or down. again, the device output the same blips now,  it concerns me that the OE is supposed to be relative to 2.*V, which is my low side, but reads 3.3V, which is my high side power.

    i hope the answer is not that this part is blown because this is in a soic package, and i have soldered thousands of these in my life with no issues before.  none like this.

     

    dave bassett

     

  • 0 in reply to david c bassett69540
    Intellectual 425 points

    hello again,

     

    here is a diagram of the connections to the tca9406:

     

     

    TCA_9406_ISSUE.pptx
  • 0 in reply to david c bassett69540
    TI__Intellectual 1930 points

     The OE pin is an input pin and therefore you should see whatever voltage the pin is tied to.. if you are seeing 3.3V on it when it is pulled to ground, unfortunately the only likely explaination is that the device is destroyed. It may not be destroyed during soldering, it could be some transient surges during power up or some other physical damage to the device.

  • 0 in reply to Akhil Nair
    Intellectual 425 points

    Hi,

     

    O.K., thanks.  the question is, if there are any rules for power up - if one side has to be powered before the other or if OE has to be low during any or either side powered up, or if it is ok to power up enabled by tying zOE to either side.  i didn't see any constraints in the datasheet.

    thanks

    dave bassett