??? 05/17/10 09:36 Read: times |
#175963 - Yes, needed for supply voltages Responding to: ???'s previous message |
I read the question as if it is needed to have capacitors for the supply voltage. The answer is normally yes. The PCB traces are not ideal - the have an impedance so there will be a pulsed voltage drop when the processor consumes power clock tick by clock tick.
On one hand, the use of supply capacitors will reduce the radiated noise from the board - remember that the PCB traces are also antennas. On the other hand, the use of supply capacitors will reduce the voltage dips, which may help with the stability in case the power supply is weak or the PCB doesn't have proper copper planes for VCC and GND and instead have long PCB traces for the supply. Besides having capacitors close to the VCC and GND pins of the integrated circuits, you also need capacitors close the the power supply - both some with low capacitance and a one or more really big one - electrolyte or similar. This helps the voltage regulator do a better job and not start to oscillate. But you should not have a capacitor (RC circuit) for the reset signal. Use a real three-pin reset driver. A RC reset gets into troubles for slowly rising voltages, or if the voltage slowly drops below allowed supply voltage and then rises again. |
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