decoupling capacitor between vcc and gnd

If you are using a linear regulator to generate 5V from 12V, that will need some decoupling for stability - see the datasheet for the regulator for recommended minimum decoupling capacitors. If you are using step-down switched regulators, it would be necessary too (again see the datasheet).

If you weren't regulating a lower voltage rail from the 12V, it would be "good practice" to include some capacitors on power lines, whether as reservoir capacitors to smooth out large switching currents, or to limit the propagation of digital/switching noise (or to provide some immunity from incoming power supply noise for sensitive circuits).

Without seeing your source material it is difficult to comment whether it is not there because somebody forgot, or for another reason (such as decouplers being built into modules, or because they simply weren't using it). But if for example the 12V is merely powering a display backlight, then it will generate relatively little noise, and not be particularly sensitive to any power supply noise.

So if in doubt, add some - but you can always provide the footprints for capacitors then not fit them if it becomes clear that they offer no benefit. Better than finding out afterwards that you should have. That would be my advice on what little information there is about what you are doing. Also some series inductors/ferrite beads of suitable current rating might be a good idea too, if you want to filter power supply lines properly!

I hope that helps

If you are using a linear regulator to generate 5V from 12V, that will need some decoupling for stability - see the datasheet for the regulator for recommended minimum decoupling capacitors. If you are using step-down switched regulators, it would be necessary too (again see the datasheet).

If you weren't regulating a lower voltage rail from the 12V, it would be "good practice" to include some capacitors on power lines, whether as reservoir capacitors to smooth out large switching currents, or to limit the propagation of digital/switching noise (or to provide some immunity from incoming power supply noise for sensitive circuits).

Without seeing your source material it is difficult to comment whether it is not there because somebody forgot, or for another reason (such as decouplers being built into modules, or because they simply weren't using it). But if for example the 12V is merely powering a display backlight, then it will generate relatively little noise, and not be particularly sensitive to any power supply noise.

So if in doubt, add some - but you can always provide the footprints for capacitors then not fit them if it becomes clear that they offer no benefit. Better than finding out afterwards that you should have. That would be my advice on what little information there is about what you are doing. Also some series inductors/ferrite beads of suitable current rating might be a good idea too, if you want to filter power supply lines properly!

I hope that helps

ty for your answer, I use the 12V for 2 LCDs (one is 2x16 character LCD and the other is a 320x240 display) and the DVI output chip. I looked into the datasheetof the regulator, it says it's optional to connect 2 10uF capacitor at both in/output side. To ensure it works I connected them.