Modern controller design is faced with a classic tradeoff. The demand for more processing throughput forces the use of controllers that are either faster, wider, or both. All other things being equal, this results in both increased power consumption and increased levels of radiated emissions.
A faster system clock increases throughput at the expense of increased power consumption and increased levels of radiation. However, the faster clock may be necessary only during time-critical processes, such as interrupt processing.
Conversely, there may be instances when a slower system clock is preferred. Memory circuits may have an access time which is longer than the typical period of a system clock. To give the memory more time to respond, a "wait" state can be used, effectively halving the performance of the system during memory access.
Likewise, a fast system clock can cause increased levels of radiated emissions. Many clocks in the past have provided for a way of dynamically varying, or "dithering" their frequencies to spread their radiated emissions over a greater bandwidth. However, these clocks have typically been relatively complex or expensive.
There is a need for a system clock which can adapt its frequency to respond to the present requirements of the processor, and provide for dithering.