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Electromagnetic Noise Reduction Slide 49

We have already discussed the fact that the main system clock can be an important source of noise as it drives the ICs in a system. The same comment holds true inside the microcontroller. One important EMI-reduction approach MCU designers can take to reduce EMI is to change the driving capability of the clock oscillation circuit to match the changing needs of the on-chip peripherals, accommodating varying operating conditions, including differences in Vcc levels and CPU frequencies. As was the case on a system level, the goal is to prevent clock signal overshoot and undershoot, and smooth the transitions. A dual-drive-mode clock oscillator circuit is used in Renesas M16C MCUs. It starts up with a larger, more powerful buffer amplifier (A) to get the oscillator started reliably, and then switches to a smaller buffer amplifier (B) when appropriate to sustain the steady-state oscillation and generate minimum EMI.

PTM Published on: 2011-11-02