Measure how many clock cycles a function takes to execute

Projects Programming
1

Hi!

I want to measure how many clock cycles a function takes to execute.
That way I would now how many MHz a program actually needs.

I tried using avr-objdump and the document "AVR 8-bit Instruction Set" to count clocks for each instruction.

Is there any better ways?
Does it exists any utilty that count instructions/clocks for a program or function?

/Olof

2

What, are you planning to put in a slower crystal?

You could call micros() at the start of the function, call it again at the end, and subtract one result from another, to get a rough estimate. Not sure how useful that really is though.

3

olof_n:
I tried using avr-objdump and the document "AVR 8-bit Instruction Set" to count clocks for each instruction.

That would only really help if the function didn't have any loops (or call other functions).

4

Thanks for the reply Nick!

Ok, so It will not work with loops, to bad.

I was curious because it would be great to know If my program could use the 8MHz internal oscillator instead of a external 16MHz crystal.
I call a multiplex function every 1000 micros and it would be great to know how much "CPU time" a have left.

5

Set a pin high before you call the function and low after. Measure the pulse width with a scope or logic analyser.

If you don't have those instruments buy one :slight_smile: (or use Nick's micros() suggestion).

Rob

6

Set a pin high before you call the function and low after.

Preferably, using direct port manipulation, to minimise overhead

7

olof_n:
I call a multiplex function every 1000 micros and it would be great to know how much "CPU time" a have left.

Take the delays out and see how fast it goes?

8

I use this kind of loop.

unsigned long start = millis();
for(int i=0; i<10000; i++)
{
   f();
}
unsigend long stop = millis();
Serial.println((stop-start)*1.6);

using 10000 calls prevents the 4usec stepsize of micros() - OK it also counts the loop increments, the compare and jumps(? ticks)

The *1.6 gives approx nr of clockcycles

suppose it takes 100 millis() to complete 10000 calls then 1 call = 160 ticks = 10 micros

9

I have a scope so I can try and measure the time or try the software solutions you have proposed.

To speed up the code I use digitalWriteFast.
I don't think that speed is an issue In my program but I was curious how many "MHz" would be enough.

Thank you all.