Rabbit 2000™ Microprocessor - UTN

With a Rabbit-based system it is easy to meet the Class B limits if proper PCB layout precautionsare observed. At Z-World, our target is to beat the Class B limit by 10 dB.3.4.2 Layout and Decoupling for Low EMIGenerally you should design with a 4 (or more) layer printed circuit board. The cost of a 4-layerboard as compared to a 2-layer board is about 30% more per square inch and generally well worthit. Although we have not attempted it, a 2-layer design would probably work for lower clock frequenciesif the ground and power nets are well gridded for power distribution (see Section 3.4.2.5on page 18).Usually a 4-layer printed circuit board has a ground plane and a power plane located as the twoinner layers and connect layers on the top and bottom layers. Components may be mounted ononly one side or on both sides. A 6-layer board places the ground and power layers as the middletwo layers and then has two routing layers both above and below. Adjacent routing layers runtraces at right angles to minimize coupling between signal traces. Sometimes the ground andpower layers are placed on the outside of the boards, but this makes debugging more difficult andcompromises the layers more since they have to be cut up for the component footprints.3.4.2.1 EMI SourcesMost EMI comes from signals that are strictly regular. The main sources are the crystal oscillator,the lines emanating from the Rabbit chip that are affected by the internal clocking of the chip, andthe actual clock or clock/2 if it is run around the printed circuit board. Address and data lines generateless EMI because there is no regular frequency on these lines since the bus cycles vary inlength, shifting the signal phase constantly. A0 is the hottest address line since it is varying mostrapidly and also has a stronger drive than the other address lines.A square wave has harmonics at odd frequencies that decline in amplitude proportional to 1/f. Asmall wire that acts as an antenna radiates more the higher the frequency of excitation. The effectivenessas an antenna increases proportional to frequency. These two effects approximately cancel,resulting in an approximately flat spectrum for a typical printed circuit board. For example,without taking precautions, it would not be unusual to have a problem with the 7th harmonic, or154.7 MHz when the Rabbit clock is 22.1 MHz. Above approximately 300 MHz, the edges are notfast enough to generate strong harmonics for typical Rabbit systems.Designer’s Handbook 13