Universal Serial Bus (USB)MCF5272 ColdFire ® Integrated Microprocessor User’s Manual, Rev. 3Freescale Semiconductor 12-37• Bypass capacitors should be connected between the Vdd and GND pairs with minimal trace length.These capacitors help supply the instantaneous currents of the digital circuitry, in addition todecoupling the noise that may be generated by other sections of the device or other circuitry on thepower supply.• Use short, wide, low inductance traces to connect all of the GND pins together and, with a singletrace, connect all of the GND pins to the power supply ground. This helps to reduce voltage spikesin the ground circuit caused by high-speed digital current spikes. Suppressing these voltage spikeson the integrated circuit is the reason for multiple GND leads. A PCB with a ground planeconnecting all of the digital and analog GND pins together is the optimal ground configuration,producing the lowest resistance and inductance in the ground circuit.• Use short, wide, low inductance traces to connect all of the Vdd power supply pins together and,with a single trace, connect all of the Vdd pins to the 3.3V power supply. Connecting all of thedigital and analog Vdd pins to the power plane would be the optimal power distribution method fora multi-layer PCB with a power plane.• The 48-MHz oscillator must be located as close as possible to the chip package. This is required tominimize parasitic capacitance between crystal traces and ground.12.5.3 Recommended USB Protection CircuitFigure 12-25 shows the recommended external ESD protection circuit for the USB.Figure 12-25. USB Protection Circuit12343301.5K333322pF22pF2 x MMBD301LT1Schottky DiodeUSB PortBZX84C3V3LT1Zener Diode 0.1μF+3.3V+3.3VD +D –
