i've seen a few circuits now where instead of saying pin 6 goes to gnd, they put a capacitor instead and then tie it to ground..
555 examples, even shift registers... why? (every example i've tried removing the cap did nothing to affect the circuit)
Have to see a specific example to see what you're referring to.
I assume you are talking about bypass capacitors. A bypass capacitor is not used "instead", it's used "in addition to". Bypass capacitors are electrically connected between the power supply and ground near the integrated circuit. (This does not change the way the IC is connected to the power supply and ground.)
When digital circuits switch, a small glitch (noise) can appear on the power supply lines. That can screw-up that chip's operation or another chip's operation. Bypass capacitors tend to smooth-out the glitches. (You can also get localized high-frequency noise or glitches in analog circuits witch can cause an op-amp to behave erratically if not bypassed.)
Often, you can remove a bypass capacitor with no ill effects. But, it's proper practice to use them!!!! Usually there is one per chip, or if there are +/- supplies, there is one for each supply on each chip. Some high-speed or "large" chips with multiple power connections will have one bypass cap for each power-pin.
For example, there are hundreds of bypass capacitors on a computer motherboard, all electrically connected identically in parallel across the power supply(s), but physically in different locations around the board. If you remove one (or several) the board will probably still work fine. But if you remove too many, the board will get flakey or totally fail.
Or you could build a prototype that works fine without the bypass caps, but when you build 100 boards, 10 of them (or half of them) don't work. Or, maybe the 1st 100 work and the 2nd batch of 100 all fail.
instead of saying pin 6 goes to gnd, they put a capacitor instead and then tie it to ground
If they do that then the pin is not connected to ground.
(every example i've tried removing the cap did nothing to affect the circuit)
That is because your tests were not good enough to see the effects of the capacitor.
If you give concrete examples then we can help.
For example pin 5 on an NE555 is connected like you say for some configurations. Connect this directly to ground and it will not work. Let it float (connect it to nothing) and it might appear to work. This is to smooth and stabilise the control voltage so if it is floating there will be noise being picked up on it. This will introduce a variation in the threshold voltage and cause jitter on the output. You will only notice this when you are looking on a scope. In some environments the pickup and hence the jitter will be larger than others. It depends on you application whether this jitter is important or not.
cjdelphi:
i've seen a few circuits now where instead of saying pin 6 goes to gnd, they put a capacitor instead and then tie it to ground.
Pin 6 of what? Are you sure you didn't read it upside down?
I suspect you're trying to generalize 1 or 2 instances of something that may or may not be related. (Hint: they probably aren't.) The functions of a 555 are completely different from a shift register chip, so it is probably best to ask about ACTUAL examples.