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TXS0108E: TXS0108E connected I2C devices

De Bug
Prodigy 220 points
Part Number: TXS0108E

Should one output of the TXS0108E only have one I2C device connected to it?

P.S: I have some boards with several I2C different devices. I just happen to notice that when VccB (5V) is connected and VccA left open the I2C devices just works fine. The I2C devices Vcc's are connected to VccB (5V) The 5V, 3.3V, SDA, GND and SCL are from an RPI3 in this case. Of-course in the final design VccA will be connected (3.3v) though curious why this is and if this makes sense and is normal behavior.

  • 0
    TI__Expert 6680 points

    Hi,

    Could you post your schematic please? One big design constraint with the TXS series of devices is signal trace length / capacitive loads.

    TI recommends careful PCB layout practices with short PCB trace lengths to avoid excessive capacitive loading and to ensure that proper one-shot triggering takes place. PCB signal trace-lengths should be kept short enough such that the round trip delay of any reflection is less than the one-shot duration. […]
    Both PCB trace length and connectors add to the capacitance of the TXS0108E output. Therefore, TI recommends that this lumped-load capacitance is considered in order to avoid one-shot retriggering, bus contention, output signal oscillations, or other adverse system-level affects.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Prodigy 220 points

    Hello,

    Thanks for the reply. Just concept testing some I2C devices with an RPI3. the RPI3 I2C port is  3.3V and the I2C devices (currently 2) run at 5V so therefor the application of the TXS0108E .The 5V & 3V power to the TXS comes from an LM1117-3.3V and 7805 regulator with the GND connected to the RPI3s GND. The 5V to the devices comes form the same 7805. The RPI is powered by it's 5V USB connector. The SDA & SCL and GND wires from RPI3's header are wired to a hand soldered and wired board with one or or more I2C devices.

    I ramped up the RPI3 frequency from RPI's default of 100000 in config.txt to 400000 and also tried to  overclock the device to 800000 without seeing a single failure after hundreds of thousands of transfers. (Did not use a logic probe to measure the actual clock though, the RPI3 could be throttling). the RPI is not used with a custom I2C bit bang . IT is usingt the standard RPI I2C driver.

    The target (future) use case is that the primary (MPU/MCU) board will have a 2.0mm or 2.54mm connector or FPC header with a very short cable to another PCB with the I2C devices and the TXS0108E itself .Both boards will be manufactured ofcourse).

    Just wondering

    1. why NOT connecting the 3,3V to the TXS also works (no errors as well), any idea ?

    2. IS the TXS designed to support more than one I2C device on the B side ?

    Thanks

  • 0 in reply to De Bug
    TI__Expert 6680 points

    Hi,

    To sort of answer both of your questions at once, the TXS0108E is essentially a pass transistor between both of the different voltage domains. I'm not entirely sure why the device is able to operate without connecting a supply rail, but I think a schematic would help clear things up there.

    As for the I2C question, yes, the TXS0108E is designed to support multiple slaves. You'll just need to make sure that your capacitive load on the B side is small so that the spec in section 6.1 of the datasheet is met.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Prodigy 220 points

    Hi, See as below,, it's pretty straightforward. Removing Va it still works.

    P.S also used this w/o the regulators,with 3.3V and 5V from RPI header.

  • 0 in reply to De Bug
    TI__Expert 6680 points

    I see,

    Are you experiencing any issues with the VCCA rail connected?

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    TI__Guru 61770 points

    TXS doesnt support Ioff partial power down mode where the IOs are in HiZ when either one of the power supply is at 0V. With Vcca floating, the Ioff would not have been guaranteed anyways.

    Without Ioff, the IOs can drive back current into the Vcca supply port and turn it on and could make it work.

  • 0 in reply to ShreyasRao
    Prodigy 220 points

    it works perfectly w/o VccA. even at overclocking speeds (that is RPI's I2C config.txt ,which may result in CPU throttling) I am now hooking the TX0108S up  to an embedded MCU and see if it has the same behavior This is the answer i was looking for, it might work but not guaranteed w/o VccA.. Thanks for the explanation.That should explain what I see, issue closed.