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SN74LVCH16T245 Tristate function

John levring
Prodigy 20 points
Other Parts Discussed in Thread: SN74LVCH16T245, SN74LVC16T245

We intend to use the SN74LVCH16T245 for test equipment purpose to interface 5V and 3.3V system.
Furthermore we need to be able to tristate the outputs (on the 3.3V side).

For some reason though, it seems impossible to tristate the outputs using the "Tri-State output-mode enable" (/OE) pins.
The Tristate function is validated using a 1 MOhm resistor connected to 2V as shown.
When the /OE pins are pulled high, the OutputVoltage is still at either low or high state. Changing state of the /OE pins has no effect on the output.
Removing the VCCB power, however, tristates the output fine (OutputVoltage approx. 2V).

Anyone got an idea why the /OE pins are non-functional?

  • 0
    TI__Mastermind 30375 points
    Hello John,

    This is likely due to the bus hold feature of the LVCH devices. The 'H' in the device name denotes this feature. Devices with bus hold will maintain the last logic level seen on the pin, so if the an output was last seen as high before being disabled, the bus hold circuitry will retain the high logic level even after the output is disabled. If you want a device without bus hold, the SN74LVC16T245 is pin for pin compatible.
  • 0 in reply to aozer
    Prodigy 20 points
    Yep, that was it - thank you!