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TXS0108E: Getting 19MHz signal on Pin1

Neeraj Verma
Prodigy 220 points
Part Number: TXS0108E
Other Parts Discussed in Thread: SN74AHCT125, LSF0108

Hi Experts,

I am using TXS0108E for 4 UART (Baud 115200) as level shifter from 5V to 3.3V.

I see most of the times I receive 19MHz clock on Pin 1, not sure where is this coming from, it makes UART to receive data continuously without having any data.

Could you please help me what could be wrong? I am blocked on this :(

Attaching circuit board snipts.  

Thanks

Neeraj

  • 0
    Guru 316920 points

    The TXS has edge accelerators that trigger when it detects the beginning of a rising edge on an I/O pin.

    I'd guess that you have long traces/cables/connectors that increase the capacitive load, which causes ringing, which can be misdetected as a rising edge. If every falling edge results in a (false) rising edge, you get oscillations. If you know the characteristic impedance of your signal lines, you can reduce ringing by adding series resisors for impedance matching.

    You could replace the TXS0108E with the LSF0108, which has no edge accelerators (and requires external pull-up resistors). But with all signals begin unidirectional, you could also use two SN74AHCT125‍s running at 5V/3.3V for up/downtranslation.

  • 0 in reply to Clemens Ladisch
    Prodigy 220 points

    Thank you so much for your valuable feedback and suggestions.