[sdiy] Tri-state outputs and CMOS inputs.

[Dave Kendall]

Thanks Florian.

That makes sense. It's an Octal 3-state latch I'm using. I just 
temporarily disconnected the 8 resistor/LED indicators from it's 
outputs, and Bingo! the circuit went crazy.
Luckily it didn't explode though .... :-)


cheers,
Dave


On Feb 20, 2009, at 16:46, Florian Teply wrote:

> Am Freitag 20 Februar 2009 16:56:22 schrieb Dave Kendall:
>> Hi all.
>>
>> I can't find a definite answer anywhere to this one - Do CMOS inputs
>> need pull-up/down resistors if connected directly to the outputs of
>> ,say, a 3-state latch - which is not connected to anything else)
>> In my experiments, it *seems* to work OK, but there are too many
>> variables in the rest of the circuit to be sure....
>>
>> My gut feeling is that CMOS inputs *should* need resistors, but it'd 
>> be
>> good to know for definite.
>>
> Short answer: it depends... ;-)
>
> Longer story: as long as the 3-state output connected to the cmos 
> inputs never
> goes to its third state (Hi-Z), everything is fine as it is, just with 
> the
> added expense for the unneeded 3-state output.
> But if the output enters hi-Z mode, the input is quasi-floating, so 
> virtually
> everything could happen: Input acting as if a logic high was applied, 
> acting
> as if logic low was applied (maybe even dependent on the level seen 
> before
> output went to hi-Z, same-as-before and opposite).
> Basically, the CMOS input would be self-biasing to somewhat around 
> half its
> supply voltage, for which behaviour is undefined. Worst case would be 
> highly
> exothermic destruction as both input MOSFETs would be conducting and 
> thus
> shorting your supply.
>
> HTH,
> Florian
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