clarification on Oscillators

[elecLear78]

Hi,

I have this clarification, in a micro controller there is an oscillator. The oscillator can be an external oscillator or a crystal. The output of this is the clock source to the micro controller. I studied that the clock is not the same for Flash memory, RAM memory, Core, Peripherals. why is it different for different modules and what is PLL? Does the PLL divide the clock or multiply.

Thanks and regards

 

[cowboybob]

Hi,

I have this clarification, in a micro controller there is an oscillator. The oscillator can be an external oscillator or a crystal. The output of this is the clock source to the micro controller. I studied that the clock is not the same for Flash memory, RAM memory, Core, Peripherals. why is it different for different modules and what is PLL? Does the PLL divide the clock or multiply.

Thanks and regards

Last question first: PLL is "Phase Locked Loop", which is an electronic scheme for:

1. assuring that the phase of a clock signal (in a microprocessor in this case) is consistant and predictable, thus assauring that the timing of actions of the processor are in complete syncrony, or happening at precisely the same moment at different parts of the processor. This is critical to the functioning of the processor

2. taking the base (primary) crystal oscillator signal and multiplying and dividing it into other frequencies (division would also involve digital dividers in the feed back loop).

Different "parts" (RAM, Core, etc.) of the microcontroller (uC) are, by design, clocked at different frequencies to suit their particular function. As you might guess, the processor of the uC is, generally, clocked at the fastest rate, primarily because it is orchestrating ALL of the various actions of the uC. Peripherals, less often called upon for action or by their very nature "slower" (like a USB, for instance) thereby require slower clock speeds.

But the entire system must be in perfect timing harmony and, thus, the need for the PLL circuit(s).