TXB0108: TXB0108E Buffer Loop Only works when I put the scope probe on it

Michael Margolese

Part Number: TXB0108
Other Parts Discussed in Thread: SN74LVC1T45

I take one of the outputs and loop it back to input and (3V -> 5V -> 3V) and get no output on the pin.

FPGA (Heartbeat output) -> TXB0108E Pin A1 (3V3) -> B1 (5V) Looped back to B2 (5V) -> A2 (3V3) -> FPGA (INPUT) --> LED FOR BLINKY

If i do not have a scope probe on B1 or B2 the light does not blink

If I probe the signal the signal appears on the scope and the led blinks

over 5 years ago

0 Emrys Maier over 5 years ago

TI__Guru* 97066 points

Hi Michael,

The TXB translator has very little output current capability - there's a 4kohm resistor in series with the output. It's only designed to send signals to low capacitance loads (like a standard CMOS input).

If you're trying to drive an LED from a 3.3V input converting to 5V, I would recommend using a unidirectional voltage translator from the LVC logic family like the SN74LVC1T45.

0 Michael Margolese over 5 years ago in reply to Emrys Maier

Prodigy 175 points

Hi Emrys,

I should be more clear. The FPGA is driving the LED. The buffer is driven by the fpga, loops back, and drives an fpga input pin.

I tested this same circuit with a simple PMOS level shifter and it works great.

0 Emrys Maier over 5 years ago in reply to Michael Margolese

TI__Guru* 97066 points

Can you provide a schematic? I don't really get what you're trying to do.

0 Michael Margolese over 5 years ago in reply to Emrys Maier

Prodigy 175 points

Note DATASHEET says that capable of driving a max of 70pF

0 Michael Margolese over 5 years ago in reply to Michael Margolese

Prodigy 175 points

0 Clemens Ladisch over 5 years ago in reply to Michael Margolese

Guru 318920 points

That is probably more than 70 pF.

And in the general case, TXB outputs are not capable of driving TXB inputs.

Do you actually need an automatic bidirectional translator in this application?

0 Michael Margolese over 5 years ago in reply to Clemens Ladisch

Prodigy 175 points

Thank you for the response ! Yes the auto sensing feature would be great in the actual application.

I doubt there is 70pF load. When I measure one end of the rca cable on the scope I get a perfect square wave.

The input cap of the buffer is not 5x more than a scope probe.

TXB outputs not being able to drive TXB inputs seems like a terrible design flaw ! Is that true TI ?

0 Emrys Maier over 5 years ago in reply to Michael Margolese

TI__Guru* 97066 points

Hi Michael,

Considering that this is an auto-bidirectional device - it is actually a requirement of the design for it not being able to drive itself. The inputs must be _weak_ so that they can be overdriven. To drive the inputs, you need a strong driver.

If you're looking for a strong driver, then you are not looking for an autobidirectional translator. We have direction controlled and unidirectional drivers that can do what you're trying.

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TXB0108: TXB0108E Buffer Loop Only works when I put the scope probe on it