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Original question:

TXB0108: GND Level Translation

TXB0108: Output latches on previous state. Misses falling edge.

Sofia R.
Prodigy 230 points
Part Number: TXB0108
Other Parts Discussed in Thread: DS34LV87T

Hello people,

After fixing the previous issue we had with this device, I need help with something kind of related.

We have a 0-2.5V signal going into the A8 pin of TXB0108. The B8 pin (0-3.3V) is connected directly (no pull-ups or pull-downs anymore) into the enable pin of a DS34LV87T.

With 0V on A8 we have 0ish volts on B8 and DS34LV87T is disabled.

With 2.5V on A8 we have 3.3V volts on B8 and DS34LV87T is enabled.

However, if A8 is disconnected and left floating, A8 goes to 0V, but B8 remains high, with DS34LV87T enabled. This is not the desired behaviour.

I have tried using pull-downs (>50k) on the A8 and B8 pins with no improvement.

My suspicion, looking at the internal diagram of the TXB0108 is that for some reason it is not detecting the falling edge of the signal present on A, hence why B stays latched high.

However, if I touch B8 with my multimeter probe, it instantly goes low and stays low. 

Any ideas on how to fix this situation? We currently have the OE pin of  the TXB0108 tied directly to 2.5V, but maybe we should try to command it instead of relying solely on the enable line for the DS34LV87T.

  • 0
    TI__Guru 58320 points
    Sofia,

    I suspect that the A8 is currently going into Hi-Z without a low transition?

    I would suggest to have the A8 programmed to actively drive from high to low to enable a high to low transition on the B8 port before the A8 going into hi-Z.

    Reason: The TXB is a weak latch which tries to retain the last known state on the ports. The B8 last active state being high, it will stay high when the input line is left floating. The input will also be held high through the feedback. In order to trigger the oneshot and have the B8 low, the A8 must be actively driven low before going Hi-Z. The weak pulldown may not be strong enough to pull the line low.