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What is the SN74LVC16T245 Port B Logic Level when Port A is in invalid state ?

Mohammed Azlum
Intellectual 835 points
Other Parts Discussed in Thread: SN74LVC16T245, SN74LVCH16T245

Dear TI Community, 

I'm using SN74LVC16T245 for translating level from 2.8V to 3.3V. 

Port A is connected to 2.8V and which is input (camera I/O signal connected). 

Port B is connected to 3.3V and which is output (connected to processor). 

When we suspend the camera, then nothing will be feed into the port A of SN74LVC16T245. This time what will be the I/O level state of Port B ?

In other words, If the input of Port A is not valid state, then what will be the logic level state of Port B ? 

Regards,

Azlum

  • 0
    TI__Expert 6335 points

    Azlum,

    There's an app note that details the LVC device characteristics. Page 18 has a section for floating inputs and the benefits of bus hold (shown below). The first paragraph shown below provides the answer to your question. The second paragraph below explains why a bus-hold device is a preferred solution to address this situation.

    My recommendation to you is to either (a) Replace SN74LVC16T245 with SN74LVCH16T245, or (a) add external pullup or pulldown resistors on the inputs.

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Intellectual 835 points
    Dear Nirav,

    Here I have one doubt:

    If I connect pull-up/pull-down in camera lines, then this may affect actual camera output ?

    Other words, Is that OK, when we connect the pull-up/pull-down in camera data bus ?

    Regards,
    Azlum
  • 0 in reply to Mohammed Azlum
    Intellectual 835 points

    Dear Nirav,

    I guess, when we use pull-up/pull-down resoistors at data bus then it is similar to the simple  level translator logic (as indicated in below image). Is it ?

  • 0 in reply to Mohammed Azlum
    TI__Expert 6335 points

    Azlum,

    Yes, having external pullup or pulldown resistors will impact the VOH and VOL output levels of the camera. I am assuming that the camera outputs are push-pull mode.

    To determine the correct resistance value to select, you would need to know the RDS(on) of the camera NMOS and PMOS transistors.

    Scenario #1 - Using an external pullup resistor [R_PU] to VCC

    This will increase the VOL level. When the camera is driving low, it's NMOS transistor is enabled and connects to GND. With the NMOS transistor in series with the external pullup resistor, you have a voltage divider on the channel. The VOL can be approximated as:

    VOL = VCC*[NMOS_RDS(on)]/[NMOS_RDS(on) + R_PU]

    You would need to select a large enough R_PU to ensure that the VOL is below the VIL(max) of the SN74LVC16T245

    Scenario #2 - Using an external pulldown resistor [R_PD] to GND

    This will decrease the VOH level. When the camera is driving high, it's PMOS transistor is enabled and connects to VCC. With the NMOS transistor in series with the external pulldown resistor, you have a voltage divider on the channel. The VOH can be approximated as:

    VOH = VCC*[R_PD]/[R_PD + PMOS_RDS(on)]

    You would need to select a large enough R_PD to ensure that the VOH is above the VIH(min) of the SN74LVC16T245.

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Intellectual 835 points
    OK. Explanation is brilliant.

    Consider I'm using "SN74AVC4T245PWR" for level translating purpose (2.8V to 3.3V).

    DIR1,2 = Low
    #OE1,2 = Low

    A port is Output (Enabled)
    B Port is Input (Hi-Impedance sate)

    This case B port is High impedance state. It does mean, B port will not drive back any High or Low ? "OR" B port is not able to drive back any High or Low ?

    Regards,
    Azlum