vsmGuy said:Come on have a look.. specially people with real designs under their belt !!
Jot down your thoughts !
Where are the advices on glitches ? Current consumption ? Noise susceptibility ?
Are these questions simple ? That's why no one answers them ?
melc said:One unused gate input to ground is better than one unused gate to VDD.
melc said:Things are simple. One gate input is better than two or three in parallel.
dmk said:I am afraid you last quation is not clear enough (at least for me).
dmk said:Anyway, to connect an input of a (N)AND gate to ground is a serious mistake (you will not damage it but it will not work at all).
dmk said:The better is to connect it to Vcc by a resistor (it will not load the driving output and the circuit will work faster).
dmk said:If you use (N)OR you must connect the unused inputs to the Ground (through a resistor - pull-down).
vsmGuy said:melc said:One unused gate input to ground is better than one unused gate to VDD.
What about (N)AND Gates where you cannot ground an input ?
What would you do then ?
Short both inputs or hook one to Vcc ?
Read this first:
https://focus.ti.com/lit/an/scla007a/scla007a.pdf
Shorting two inputs means double parasitic capacitance. If you need to transfer a clk at the gate transfer frequency limit, then you don't connect two inputs together.
If you have some HC logic connected to your PIC, than you can connect it in the easiest way for routing, PIC is lazy
melc said:Things are simple. One gate input is better than two or three in parallel.
My point exactly !
Jot down your thoughts !
I told that consider them as run of the mill 74HC parts ! I have even the part numbers written beside them !
Didn't noticed untill you pointed.
What would you do if you had these options while making a digital design using exclusively these parts ?
I know the question I have asked in this thread is nothing special.. but something tells me, people who have designed digital designs using such ICs hava whole lot to tell on this very simple issue..
I would like to hear from them !
laktronics said:Unless I am left with only any one of the other options, I would prefer circuit U5:A
laktronics said:3. Minimum cell configuration from the given choices so that you can get more gates in a given package.
laktronics said:4. I prefer not to vote because I feel that you got more out of me than me from your post.!!!
dmk said:"There is an idea that power planes are susceptible to noise and not ground planes."
They both are susceptible to noise (that is why you can often see "power rails"). Sometimes it's possible that not the whole current flows through the Vcc (if you have more than one supply voltages - say +5V, 3.3V, +-12, etc.). But it will always flow through the ground. That's the reason the ground is more important than the power supply.
dmk said:"Could you tell me "it will not load the driving output" a little more clearly ?"
Each output has sa called "fanout" - the (maximum) number of inputs it can drive. So, first it's good not to load the output if there is no need. Second, each input (as well as the wire, connected to it) has a parasitic (stray) capacity. Each time the output change its state, it must charge or discharge it. In most cases it is not important (in low freqs). Any way, the output must insure different current for 1 or 2 inputs it has to drive. Sometimes this causes troubles.
dmk said:"Would I be crazy if I said that would in fact introduce the possibility of noise creeping in ?"
No, you are not crazy saying this. It is correct. Nevertheless it is better (not obligatory) to use pull-up/down resistors (just to prevent the damage of inputs when the device is power on).
dmk said:"There is an idea that power planes are susceptible to noise and not ground planes."
They both are susceptible to noise (that is why you can often see "power rails"). Sometimes it's possible that not the whole current flows through the Vcc (if you have more than one supply voltages - say +5V, 3.3V, +-12, etc.). But it will always flow through the ground. That's the reason the ground is more important than the power supply.
A good designed ground plane will allways have lower impedance than a V+ supply (anyone will be that one), just because the V+ ( or V-) comes from a power supply with miliohms output impedance in the best situation, while the ground plane will be allways the reference.
You can't say that "ground is more important than power supply". Both are important as well, that's why has been invented the ZBC technology where the isolation between ground and power plane is 1mil (ZBC1000) or 2mil (ZBC2000) creating distributed filtering capacitors.**broken link removed**
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