Re: 74HCT outputs in paralell

David L. Jones wrote: 
Tim Hubberstey wrote:

Rob wrote:

"David L. Jones" <altzone@xxxxxxxxx> wrote in message
news:1125490862.084317.292540@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Actually, it can be good practice to connect the inputs via high value
resistors (say 1M) to ground (or Vcc) instead of just tying them hard
to the rail. In this case it means that you can use the gates at a
later stage without cutting any PCB tracks, simply wire your signal up
with some mod wire. If cost and board space etc isn't an issue then it
can come in handy.

Dave :) 

Thats a good idea using a p/up p/down resistor, I should do that.
cheers
rob


It is a good idea, but 1M is too high a value for the resistor. You have
to consider the voltage drop developed across any pull up/down caused by
the input leakage current. For HC/HCT, this is (from memory) 10 uA, so
100k is the largest value I would consider. Besides, the "harder" you
pull the input to the rail, the less chance there is that the leakage
voltage at the input will put any of the transistors in the gate into a
partially conducting state. I usually use 1k for this since most
assembly houses have reels of this value in stock and they tend to be
dirt-cheap.



It's around 0.1uA max for HC stuff, so 1M is fine. Always check the
data*** if in doubt though. 100K is a popular value for this task for
CMOS logic.

Actually, we were both wrong. I just looked up a couple of data sheets and it's 1 uA max, across temperature. I remembered the 100k correctly though. This gives a maximum voltage of 100 mV, well within the 800 mV worst case for a "0" and low enough to keep "off" transistors off.

1K is way too low!
If you do the mod (which is why you put the resistor there in the first
place) and drive the input from another gate with 5V logic, that's 5mA
you are throwing down the drain. These "large" current spikes could
have follow-on effects into other circuitry too. If you did use 1K then
you'd remove the resistor.
Often you'd pick a resistor value that you already had in your BOM, so
it doesn't mean another line item.

We have different opinions on how to do mods. I assumed that the resistor will be removed when the board is modified, then the resistor value has no impact on current drain. Even with a 1M, you are still throwing away up to 5 uA per input if you don't remove the resistor. Besides, this way you have a nice pad to solder your jumper wire to.

--
Tim Hubberstey, P.Eng. . . . . . Hardware/Software Consulting Engineer
Marmot Engineering . . . . . . .  VHDL, ASICs, FPGAs, embedded systems
Vancouver, BC, Canada  . . . . . . . . . . . http://www.marmot-eng.com
.

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