2011/5/26 Ruben J=F6nsson > > > > 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 > much > > current flowing through them anyway. The LEDs are the biggest current > drains > > on there. Also, the drains on the MOSFETs, so I'll make those a bit > bigger > > too. > > > > With a digital design (like much of this one) you also need to take into > account the switching currents of transistors inside the digital chips > (MCU, > FPGA, 74 logic, drivers...). Especially for chips which are working with > clock > signals (changes state with the clock) such as an MCU. These chips have a > lot > of transistors switching every clock and it is when they are switching th= at > they draw current. This means that you have a current spike for every clo= ck > transition. > > If you have long and thin power/ground tracks for these chips, the tracks > will > look like a relative large resistance between the current supply and the > chip. > This resistance varies with frequency which means that at certain > frequencies > you will have a higher resistance and at other frequencies you will have > lower > resistance. > > Whenever you have a resistance in a current path, you also have a voltage= .. > The > higher this voltage becomes, the more trouble it can cause. One of the > trouble > it can cause is to make the trace an unintentional RF transmitter. Anothe= r > is > to make a difference in the ground voltage (or any reference voltage) > between > two chips on the same board which could cause false transitions of logica= l > inputs. > > Remember that the switching frequencies (clock) can be quite high and sin= ce > this mostly is a square wave it contains a lot of overtones with much > higher > frequencies and it is the higher frequencies that can cause troble here > (remember that the resistance is varying with frequency and usually is mu= ch > higher at higher frequencies). > > In my experience, when a circuit transmitts RF unintentionally (emitting > diturbances), it also works as an RF receiver (has low immunity against R= F > disturbances). The received RF (either conductively via connected wires o= r > over > the air) is then converted to voltages in the tracks on the board which c= an > make the board malfunction. Such an RF transmitter can be a relay switchi= ng > a > load. > > How do we handle these problems then? The first thing is to make a > groundplane > which ensures that you have very low resistance over the entire frequency > range > between the current sources and the chips and between the chips themselve= s. > This way the ground level is the same for all chips on the board and all > chips > sees the digital signals connected between them at the level it was > supposed to > be (the same for the transmitter as for the receiver), which means that > there > will be no false transitions. > > The other thing is to make sure that the ICs which need a lot of current > when > its internal transistors are switching, has this current available close = to > the > power and ground pins, with a low resistance for the entire frequency > range. > This is done by making the power tracks as wide as possible and by using > decoupling capacitors. The decoupling capacitor acts as a tiny current > reservoir, which the chip can draw current from in very short bursts when > it > needs it. If the decoupling capacitor is placed close to the power and > ground > pins and the tracks between them are wide enough, the high switching > currents > are limited to those short tracks. Since not all current can be drawn fro= m > the > decoupling capacitors and since they need to be recharged between the > current > bursts, you also need wide tracks between the power supply and the > decoupling > capacitor. > > Also remember that when a chip draws a lot of current on the power pins, > the > same amount of current is going through the ground pins. This is why we > need to > have the decoupling capacitor as close to the power and ground pins on th= e > same > chip as possible (a ground plane helps here). Otherwise you will have a > long > way for the high current to travel on the board. This long way is called = a > current loop which, the longer it is the more trouble it can casue. In > other > words, keep the current loops as short as possible. > > In short (which did become much longer than I intended), this is why you > need > big fat tracks for the power supply traces on your board. It is also the > reason > you need a good decoupling for the digital chips. > > This becomes more important when you make boards professionally since the= se > have to comply with certain EMC (ElectroMagnetic Compatibility) rules. > These > rules says that your electronics may not emit RF energy over a certain > level at > certain frequency ranges (both conductively and over air) and it must als= o > be > immune to a certain level of RF energy thrown at it at certain frequency > ranges. This is really also important when making board unprofessionally. > Many > times you hear that something dosen't work and noise or RF disturbance is > blamed (a CPU gets reset for example). Most of the time I think that it i= s > more > due to bad board design casuing too low immunity thresholds for some > frequencies than a very high disturbing signal. > > /Ruben > Wow, okay. Thicker power/ground tracks it is. What thickness/clearance do you recommend? --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .