FreeLists / si-list / [SI-LIST] Re: Regarding Pull down

["Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>]

>         I wanna ask the question that in normal applications when
> inputs are
> not needed for test reasons, is it OK to tie the un-used inputs of TTL
> or
> NAND devices directly to Ground or one must always need to use the
> pull-down
> resistors.
 
If you never ever need to pull an input pin high, I think there is no
reason why you cannot directly ground it.

If it is a bidirectional pin, you should make sure it can't spend any
time with the output enabled (say, when powering up or down).

With some true TTL families, and unused inputs that you wanted to appear
High, there was a reason for using a resistor and not tying them
directly to VCC.  I think it was related to the absolute maximum input
voltages.  If I remember correctly, the supply voltage on the VCC pin
was allowed to reach +5.5V (or more?), but input pins could not go above
+5.25V.  Using a resistor on such pins, as instructed in the databooks,
limited the input pin current should a transient on the supply voltage
exceed +5.25V.  But this does not apply to inputs that you want
grounded.

Regards,
Andy




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