74AC16244: Unused inputs pins

Victor Borges

Prodigy 90 points

Part Number: 74AC16244
Other Parts Discussed in Thread: 74AC16245

Could I tied the unused inputs to GND instead to tied to VCC through a pullup resistor?

Thanks.

over 4 years ago

0 Chad Crosby over 4 years ago

TI__Expert 6680 points

Hi Victor,

Yes, you may tie these to GND. While the datasheet specifically states that the inputs must be tied high, the purpose of this is so that any unused pins are not left floating.

The below app note goes into detail about the implications of floating inputs:

http://www.ti.com/lit/an/scba004d/scba004d.pdf

Thanks!

Chad Crosby

0 Victor Borges over 4 years ago in reply to Chad Crosby

Prodigy 90 points

Thanks, Chad for your answer. I usually tie the input pins to GND, but I was concerned with the datasheet's suggestion to tie to VCC. Do you know why?

0 Clemens Ladisch over 4 years ago in reply to Victor Borges

Guru 283530 points

It appears this footnote was copied from a TTL datasheet (where a high input wastes less power).

CMOS devices (like this) do not care whether an input is low or high, and a resistor is not needed due to the high input impedance.

0 Chad Crosby over 4 years ago in reply to Victor Borges

TI__Expert 6680 points

I'm not entirely sure as to why the DS specifies this, as the 74AC16245 (a very similar product with another tri-state buffer at the output) specifies either logic high or low (see page 3 of its DS) for its \OE pin.

Thanks!

Chad Crosby

0 Victor Borges over 4 years ago in reply to Clemens Ladisch

Prodigy 90 points

That is possible. Thanks Clemens for your explanation.

0 Victor Borges over 4 years ago in reply to Chad Crosby

Prodigy 90 points

Maybe this is just a mistake in the DS like Clemens suggested. Thanks for your attention and your answers, it was very helpful.

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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74AC16244: Unused inputs pins