>=20 > Couldn't afford to make them 0.050 inches. The default net class is 0.013= " > width, 0.013" clearance. I made the GND net 0.020", 0.015". There's not m= uch > current flowing through them anyway. The LEDs are the biggest current dra= ins > on there. Also, the drains on the MOSFETs, so I'll make those a bit bigge= r > too. >=20 With a digital design (like much of this one) you also need to take into=20 account the switching currents of transistors inside the digital chips (MCU= ,=20 FPGA, 74 logic, drivers...). Especially for chips which are working with cl= ock=20 signals (changes state with the clock) such as an MCU. These chips have a l= ot=20 of transistors switching every clock and it is when they are switching that= =20 they draw current. This means that you have a current spike for every clock= =20 transition.=20 If you have long and thin power/ground tracks for these chips, the tracks w= ill=20 look like a relative large resistance between the current supply and the ch= ip.=20 This resistance varies with frequency which means that at certain frequenci= es=20 you will have a higher resistance and at other frequencies you will have lo= wer=20 resistance. Whenever you have a resistance in a current path, you also have a voltage. = The=20 higher this voltage becomes, the more trouble it can cause. One of the trou= ble=20 it can cause is to make the trace an unintentional RF transmitter. Another = is=20 to make a difference in the ground voltage (or any reference voltage) betwe= en=20 two chips on the same board which could cause false transitions of logical= =20 inputs. Remember that the switching frequencies (clock) can be quite high and since= =20 this mostly is a square wave it contains a lot of overtones with much highe= r=20 frequencies and it is the higher frequencies that can cause troble here=20 (remember that the resistance is varying with frequency and usually is much= =20 higher at higher frequencies). In my experience, when a circuit transmitts RF unintentionally (emitting=20 diturbances), it also works as an RF receiver (has low immunity against RF= =20 disturbances). The received RF (either conductively via connected wires or = over=20 the air) is then converted to voltages in the tracks on the board which can= =20 make the board malfunction. Such an RF transmitter can be a relay switching= a=20 load.=20 How do we handle these problems then? The first thing is to make a groundpl= ane=20 which ensures that you have very low resistance over the entire frequency r= ange=20 between the current sources and the chips and between the chips themselves.= =20 This way the ground level is the same for all chips on the board and all ch= ips=20 sees the digital signals connected between them at the level it was suppose= d to=20 be (the same for the transmitter as for the receiver), which means that the= re=20 will be no false transitions. The other thing is to make sure that the ICs which need a lot of current wh= en=20 its internal transistors are switching, has this current available close to= the=20 power and ground pins, with a low resistance for the entire frequency range= ..=20 This is done by making the power tracks as wide as possible and by using=20 decoupling capacitors. The decoupling capacitor acts as a tiny current=20 reservoir, which the chip can draw current from in very short bursts when i= t=20 needs it. If the decoupling capacitor is placed close to the power and grou= nd=20 pins and the tracks between them are wide enough, the high switching curren= ts=20 are limited to those short tracks. Since not all current can be drawn from = the=20 decoupling capacitors and since they need to be recharged between the curre= nt=20 bursts, you also need wide tracks between the power supply and the decoupli= ng=20 capacitor. Also remember that when a chip draws a lot of current on the power pins, th= e=20 same amount of current is going through the ground pins. This is why we nee= d to=20 have the decoupling capacitor as close to the power and ground pins on the = same=20 chip as possible (a ground plane helps here). Otherwise you will have a lon= g=20 way for the high current to travel on the board. This long way is called a= =20 current loop which, the longer it is the more trouble it can casue. In othe= r=20 words, keep the current loops as short as possible. In short (which did become much longer than I intended), this is why you ne= ed=20 big fat tracks for the power supply traces on your board. It is also the re= ason=20 you need a good decoupling for the digital chips. This becomes more important when you make boards professionally since these= =20 have to comply with certain EMC (ElectroMagnetic Compatibility) rules. Thes= e=20 rules says that your electronics may not emit RF energy over a certain leve= l at=20 certain frequency ranges (both conductively and over air) and it must also = be=20 immune to a certain level of RF energy thrown at it at certain frequency=20 ranges. This is really also important when making board unprofessionally. M= any=20 times you hear that something dosen't work and noise or RF disturbance is=20 blamed (a CPU gets reset for example). Most of the time I think that it is = more=20 due to bad board design casuing too low immunity thresholds for some=20 frequencies than a very high disturbing signal. /Ruben =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D Ruben J=F6nsson AB Liros Electronic Box 9124, 200 39 Malm=F6, Sweden TEL INT +46 40142078 FAX INT +46 40947388 ruben@pp.sbbs.se =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .