15 Sep
2004
15 Sep
'04
12:40 p.m.
>>
>> Thim makes me conclude that in order to correctly measure
>> stand by current consumption on any CMOS circuit, you
>> _must_ tie all unused inputs to either Vdd or Vss,
>> whichever makes most sense.
>>
>> If the input of a CMOS inverter is left floating, there is
>> a good chance it will stay somewhere in the middle between
>> high and low and both transistors will be half open for
>> current to flow all the way from Vdd to Vss. If the input
>> is tied to Vdd or Vss, then at least one of the transistors
>> will be shut off, and very little current will flow. This
>> is all there is to CMOS low static current con- sumption.
>>
The above is quite correct. Over current is merely one of the effects of
Leaving CMOS inputs floating. What gets real interesting are the
bi-directional pins which
Need to be terminated when they are inputs, but not HARD tied when they are
outputs.
This offten requires some "scaffolding" curcuitry to power up a board
"outside of its native habitat".