Effects of Over Clocking??

Only if there is some known reference signal, such as a 32768 Hz watch crystal or 50Hz mains etc. signal would be available.

However, I agree completely with Grant that it would be stupid if the processor would reprogram the PLL.

While it might make some sense to limit any overclocking, it would be very stupid, if I wanted to underclock a system either to save power or enable some x86 processor to run at 70+ C ambient temperatures and the PLL logic would force the nominal clock speed.

Paul

In general, the answer is no. The only way a processor could know what frequency its clock has is if it has an independent timing source with a known frequency.

Is it possible to underclock a processor?

Some processors have a (non-zero) minimum clock frequency, either because they are not static or because they use something like a PLL with a limited guaranteed operating frequency range to generate the clock, so yes, you could violate that datasheet specification.

Best regards, Spehro Pefhany

Pretty often there is a minimum clock frequency requirement for the peripherals like flash/eeprom or ADC.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

Sure. Some processors have a minimum allowed clock, some you can clock all the way down to DC. Many of the processors aimed at the embedded market can be clocked as slow as you like. I know a lot of people running "16MHz" processors at a few tens or hundreds of KHz. 32KHz is pretty common for low power applications.

Yes, as long as you don't go below the minimum requirements. Some processors are fully static (i.e., you can stop the clock entirely), while others require a minimum speed. Microcontrollers with PLLs will often have minimum requirements, but you can set the PLL for low speeds in software. Another thing to watch for are dynamic ram, which has minimum speed requirements.

Underclocking is often done to save power. It can also be useful if you are running in extreme environments, such as high temperatures - processors will typically run well above their specified temperature range if they are underclocked.