What does CPU frequency represent in an Arduino board?

Question

I'm new to Arduino and microcontrollers.

I was studying the specs and found that even the same board may have different frequencies with different input voltages (3.3V vs 5V). So the question is, what does frequency represent? Does it represent how many lines of assembly code it's able to run? Or the maximum PWM frequencies it's able to output?

A further question would be, if I'm looking for a board for a specific project, how do I decide which frequency I will need a priori, instead of trying everything out and see which one works?

What makes me more confused is that when it comes to computer CPUs, it seems that lower frequency CPUs can actually run faster than higher frequency ones (e.g. Intel). So how do I actually know how fast a microcontroller can run?

Answer 1

By frequency we mean the frequency of the CPU clock. Say your Arduino Uno runs on 16 MHz, which is 16,000,000 Hertz. That means there are 16 Million clock cycles per second. The CPU executes the byte-code of the program. One Assembler instruction can actually take any number of CPU cycles to execute, usually between 1 and 4 cycles for simple stuff, and a little bit more heavy arithmetic and writing to memory. So it's a rough estimate of how many "lines of assembler" (that is, byte-code instructions) it can run per second. A measurement which is a little bit better is the "MIPS" value, the "Millions of instructions per second". There are other benchmark types for CPUs which are more accurate.

If you take at the datasheet for the AVR microprocessor architecture, you can see the cycles that each instructions needs: (link: http://www.atmel.com/images/Atmel-0856-AVR-Instruction-Set-Manual.pdf)

So for an ADD Rd, Rr instruction, an AVR CPU needs 1 clock cycle.

Take a desktop Intel CPU for example. It's common these days that they have a clock frequency of 2 GHz or more, which is 2 Billion cycles per second compared to the 16 million cycles per second on the Arduino's AVR CPU. So the Intel CPU beats the Arduino by far. Then again, the Arduino is designed for completley different stuff - it's a small microcontroller with low overhead, runs no OS etc. The use-case for such a CPU (and the architecture) is just different, which makes comparing them unjustified. There many other factors in play, like multi-core CPUs (4 Intel CPU cores vs. 1 AVR) and command pipelining, the speed of your memory / RAM, etc. It's really hard to compare a CPU to another one in every use-case possible, but for "general purpose computing", the Desktop CPUs (AMD, Intel, x64 architecture) far outruns the processing power of a mere Arduino AVR CPU.

I hope this clears up some confusion.