SN74AVC32T245: SN74AVC32T245 unused pin

user4758819

Part Number: SN74AVC32T245

Should all unused pin be held at VCCI or GND?

According to datasheet, unused input pin must be held at VCCI or GND, however, there are no description for unused output pin.

I think it is the same thing for unused output pin, because output pin will be changed to input pin by DIR pin state.

over 8 years ago

0 Clemens Ladisch over 8 years ago

Guru 318050 points

An unused input pin must be held at either V_CC or GND.
An unused output can be open, or can be driven to the same value that the output itself is driving, but it must not be held at the opposite value (that would short Vcc to GND, and probably exceed the current rating).

If a pin is used as both input and output, and you do not know its value, connect it to either V_CC or GND with a resistor that is large enough to limit the current; use 10 kΩ or larger. If one state is more likely, connect the resistor to that power rail so that no current flows in that case.
(A bus-hold circuit would also work and would save power, but is more complex.)

0 user4758819 over 8 years ago in reply to Clemens Ladisch

Prodigy 40 points

Thank you for your support. I guess unused output can be connect to GND, is it correct?

0 Clemens Ladisch over 8 years ago in reply to user4758819

Guru 318050 points

I described all cases in my previous answer. I do not know whether your secret circuit is one where it is allowed, or one where it fries the chip.

0 user4758819 over 8 years ago in reply to Clemens Ladisch

Prodigy 40 points

Sorry for my poor explanation and English.
I think I do not catch the meaning of "but it must not be held at the opposite value".
When unused input is held at GND, unused output can be connect to GND.
Is my understanding correct?

0 Clemens Ladisch over 8 years ago in reply to user4758819

Guru 318050 points

Yes, it both I/Os are connected to GND, then they never have the wrong value, regardless of which one is input or output.

0 user4758819 over 8 years ago in reply to Clemens Ladisch

Prodigy 40 points

I got it. Thanks a lot!

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SN74AVC32T245: SN74AVC32T245 unused pin