Relation between cycle instruction and clock frequency !!

[noutche2008]

Posted on May 05, 2015 at 01:14

Hi,

Let’s start with this example :

Let’s suppose I work with an STM32F407VGT6 microcontrollers which his System core clock is set at his maximum frequency which is 168MHZ.

According to the ARM website the CORTEX-M4 assembly instruction

MOV Rd, <op2>

takes one cycle to be executed. That’s mean this instruction is executed in 5.95ns??? is that correct ??

here’s how I did :

168MHZ = 168 000 000 pulses / seconde

So 1 pulses takes 1/168 000 000 = 5.95ns to be executed

thx.

[Tesla DeLorean]

Posted on May 05, 2015 at 01:34

Basically that's how it works, things get a little murkier with slower buses, write buffers, FLASH memory and caches, etc.

[noutche2008]

Posted on May 05, 2015 at 02:18

thx for your answer.

If we take into account the frequency buses, flash latency, float point unit... etc this will give us the DMIPS value ?? right ?

[Tesla DeLorean]

Posted on May 05, 2015 at 02:30

http://en.wikipedia.org/wiki/Dhrystone

suggests a synthetic benchmark containing no floating point math, as I recall. I guess if you worked your way through all the instructions in the benchmark, accounting for all the cycles, and contributions of the system you could come up with a number.