• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 22 subscribers
  • Views 343 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TXS0108E: RESET Issue - TXS0108E

Amol Bagul
Prodigy 90 points

Part Number: TXS0108E
Other Parts Discussed in Thread: TXU0304, TXU0104, TXU0204

Dear TI Support Team,

I am using the TXS0108EPWR level shifter in a design where a reset signal BBRESET originates from an MPC555 microcontroller and is interfaced to a 3.3 V CPLD input.

I would like to confirm the suitability of TXS0108E for this application and understand its behavior under the following conditions.

System Details:

Input signal BBRESET from MPC555 is 5 V logic
An external pull-down resistor of 1 k ohm is present on the signal line
TXS0108E is used for 5 V to 3.3 V level translation
The CPLD input requires a valid 3.3 V logic HIGH
OE pin is configured as per datasheet recommendations

Observed Behavior:

The signal does not reach a valid HIGH level at the TXS0108E output
Even at the input side, the expected 5 V level is not observed
The signal appears to be held LOW or at an intermediate voltage

Expected waveform - 

expected waveform.png

actual waveform -

actual wf.png

Questions:

Is TXS0108E suitable for translating a reset signal with a strong external pull-down of 1 k ohm
How do the internal pull-up resistors of TXS0108E interact with such a strong pull-down
Can TXS0108E reliably drive a HIGH level under these conditions

Could you help analyze the reason for this behavior

Also, when an external 5 V is applied directly to the reset pin, the output shows 3.3 V at the level shifter output after isolating the MPC555

Your guidance will help us determine whether a design modification is required

Thank you for your support

Best regards
Amol Bagul

  • 0
    TI__Guru 59811 points

    Hi Amol,

    The strong pulldown will not allow the VOH to achieve levels closer to VCCO. This app note covers this use case in detail, and it is suggested to remove the external pulldown to ensure proper functioning of the device: www.ti.com/.../scea054a.pdf

    Regards,
    Jack 

  • 0
    Prodigy 90 points

    Hello Jack,

    Does the same logic will apply at the input side too?

    The pull down is placed at the input side of the level shifter. Refer the sch with 1K pull down placement.

    Also, please note the when applied the external 5V by isolating it gives the input & output at level shifter as expected. 5V & 3.3V resp.

  • 0 in reply to Amol Bagul
    Guru 325710 points

    The TXS has pull-up resistors at all pins. You should not use pull-down resistors with the TXS. If you do use a pull-down resistor, it must be much stronger than the internal 4 kΩ pull-up to be able to override it; your 1 kΩ resistor is too weak.

    For unidirectional signals, you should use unidirectional translators like the TXU0104/TXU0204/TXU0304.

  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Then how is 5 V observed at the level shifter input and 3.3 V at the output when 5 V is applied externally, with the MPC555 isolated and the 1 kΩ pull-down still in place?

  • 0 in reply to Amol Bagul
    TI__Guru 59811 points

    Hi Amol,

    Is it possible that the external 5V source is able to drive more current through the 1kohm pulldown over the previous implementation? Additionally it would be helpful to remove the strong external pullups/ leverage the TXU0x04 as Clemens' suggested as a more viable solution.

    Regards,

    Jack