Re: 74HCT outputs in paralell

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.
--
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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