[sdiy] CMOS clock inputs.

[Scott Gravenhorst]

Dave Kendall <davekendall at ntlworld.com> wrote:
>Thanks Ken.
>
>Will try that now...
>
>cheers,
>
>Dave.
>
>on 1/4/06 12:33, Ken Stone at sasami at hotkey.net.au wrote:
>
>> 
>>>> do you have a pull-down/pullup resistor?
>>>> Ken
>>> 
>>> Oops!
>>> 
>>> The clock advances on a Low-High transition, so I guess it should be
>>> connected between CLK and GND. What would be a good value to start with?
>>> supply is 0V +15V.
>> 
>> 10k or 100k. It doesn't really matter.
>> 
>>> I've had a look in some text books, and an unconnected TTL output floats up
>>> to HIGH, but it doesn't mention what CMOS inputs and outputs do...
>> 
>> CMOS are capacitive - the float, and are affected by the smallest stray
>> field, so can be quite unpredictable.

I will add that "unpredictable" means from moment to moment.  Often a floating input causes
it's associated output to toggle wildly - noise.  This noise can be very high frequency
causing the IC to heat up some and could also inject noise elsewhere into your circuit.  It
is wise to ground (or tie high) any unused CMOS inputs for this reason.  It is the high
input impedance (millions of ohms) of CMOS that contributes to this problem (and at the
same time makes CMOS desirable for certain things).

>> For a rising edge, I would use a pull-down resistor. I would also put at
>> least 1k between the input and the switch (remember that it will form a
>> voltage divider with the pulldown, so don't over do it). The switch can then
>> simply disconnect/connect to +15 with no zero volt connection needed because
>> the pulldown is already doing this.
>> 
>> Ken
>> 
>> _______________________________________________________________________
>> Ken Stone   sasami at hotkey.net.au or sasami at cgs.synth.net
>> Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
>> Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>
>> 
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.