I am laying out PCB for a stopwatch project that I made couple of months ago. I seem to have taken care of the only sensitive part i.e. crystal, since it is time measuring device. But I also see that in other projects, where we have 16x2 character LCD, that when I connect CPU there's few mVpp of noise at pins 15-16 of LCD(These are the pin for LED backlight) since I don't think this noise is due to LED backlight where is it coming from. I measured it using ground spring between pin 15-16. I was told that since that part is drawing high current (20-30 mA) the transient in other part of circuit is reflected "more" here.
Is this true.?
I checked the power line at other points and I didn't see as much noise as I see at pin 15.
This is the layout I am doing, still not complete.
U3 is my CPU. LCD Will be connected at J6,j7,j8. J9 is the point where I will connect A boost converter(External PCB). So Is my Crystal n caps layout correct, and How should I power my LCD, should the trace be very small, if yes then how much(any math), should I share power line between CPU and LCD or should I use star connection for power as well.
Also regarding the Decoupling caps on vdd of IC. Vdd pin is pin 20 and gnd is pin 10, they are as far away as it could be. Should I connect second pin of cap to pin 10 or to the power supply ground since it's closer to it.
Edit: Added Semi-Completed Layout
Thanks for your suggestions everyone. Above is top view of my semi-complete layout. Trace width for pwr and gnd is 40 mil , and logic is 12 mil and for my 20 Hz pulse output it is 25 mil. Below is it's bottom view:
I wanted to ask one more thing regarding Decoupling caps: I have read in a datasheet of an OPamp to connect 0.1uF and 1uF to IC's Vcc. Along the same lines I have connected like 0.1uF and two 1uF to the LCD's Backlight pin, Is it worth doing it or a single 2.1 uF will do the job similar to them. The whole idea for distributing Caps as 0.1 and 1uF is to have smaller cap handle fast transients and 1uF to provide bulk charge. Am I going correct. Also if you see for IC U4 I have connected Decoupling to gnd through a via, and kept via as close to cap as possible, is it enough or is it going to haunt me after production.?
Thank you very much.