SN74LVC125A open input pin

Ryuji

Guru 20090 points

Other Parts Discussed in Thread: SN74LVC125A, SN74ALVC125, SN74LV125A

Hello,

In the customer , the SN74LVC125A input is left floating some times.
Because, they are using the cable at the input. Also, the cable is disconnected some times.
However, since the output of SN74LVC125A is connected to FPGA and they can correct, they can accept the false output signal from the SN74LVC125A

Their horror is that the device was got damaged also heat up the device.
Are there any possibility of their horror when the input is left floating.

Please see the following as their operation case.

(1) Cable is disconnected and SN74LVC125A powered ON
(2) Cable is disconnected and SN74LVC125A powered ON, then the cable is connected
(3) Cable is connected and SN74LVC125A powered ON, then the cable is disconnected
(4) Cable is connected and SN74LVC125A powered ON, then the cable is disconnected and then the cable is reconnected.

The input signal come from other SN74LVC125A output signal.

Best Regards,
Ryuji Asaka

over 10 years ago

0 Clemens Ladisch over 10 years ago

Guru 326760 points

See Implications of Slow or Floating CMOS Inputs (SCBA004). To summarize: don't do it.

To avoid the floating input, use a weak pull-up/-down resistor, or build a bus-hold circuit (SCLA015) from a buffer and a resistor.

0 Ryuji over 10 years ago in reply to Clemens Ladisch

Guru 20090 points

Hello Clemens san,

Thank you for the reply.
The customer boards is already fixed.
Are there any same pin assign device which can be used floating input?

VCC is 2.5V,
Bit# is 4
3 state output

I couldn't found the device which has above requirement ans bus-hold function.

Best Regards,
Ryuji Asaka

0 Clemens Ladisch over 10 years ago in reply to Ryuji

Guru 326760 points

There is no such device. They have to bodge pull-up resistors onto the boards.

0 Ryuji over 10 years ago in reply to Clemens Ladisch

Guru 20090 points

Hi Clemens,

OK. I understood.
How about TTL input logic?
SN74ALVC125 and SN74LV125A input is LVTTL.
Is there same risk with CMOS input device?

Best Regards,
Ryuji Asaka

0 Clemens Ladisch over 10 years ago in reply to Ryuji

Guru 326760 points

Both ALVC and LV are CMOS families; the "LVTTL" describes only the voltage levels, not the actual implementation.

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SN74LVC125A open input pin