Calculating Execution Time for Code with Single Loop

Introduction

Procedure in Calculating Execution Time

• The clock frequency of the AVR microcontroller the code is running on.
• The information sheet that tells the number of cycle each instruction requires to execute.

Once you have the above information follow the steps below to calculate the execution time:

Steps to calculate ET

1. Determine the period of the microcontroller clock using the equation:
   Period (T) = 1/Frequency(f)

2. Determine the total number of cycle (C) to execute the piece of code given. This is done by determining the number of cycle for each instruct to execute in the code and adding them. The number of cycle each instruction takes to execute is found in the datasheet for the micro-controller. This require abit more work than in tutorial ET1. (see example below)
3. Calculate the execution time for the piece of assembly code using the the following equation:
   ET = T * C

Example Question & Solution

Question

Calculate the execution time for the following piece of AVR assembly code given that the code will be running on an Atmel AVR 8-bit micro-controller being clocked by a 4MHz oscillator.

                      LDI   R16, 5 
Again:                DEC   R16
                      NOP
                      BRNE  Again
                      NOP
                      NOP

Solution

Step 1 - Determine the Period (T)

Step 2 - Determine Total Number of Cycles for execution (C)

;                    Instruction                         # cycles to execute
                      LDI   R16, 5   ;                            1
Again:                DEC   R16      ;-----|
                      NOP            ;     |                      A
                      BRNE  Again    ;-----|
                      NOP            ;                            1
                      NOP            ;                            1
;                    _______________________________________________________
;                    Total # of cycles for execution (C) =      3 + A 
 
 

The total number of cycles for the execution of the code is given as (3 + A). In calculating the total number of cycles for execution, in this case, it must be taken into consideration that the block of code labeled A is executed more than once because it forms a loop.

What is done here is that we look at the block of code which construct the loop as a single instruction for which we will determine the number of cycle for execution next.

Calculating the number of cycles for block A

;                   Instruction                         # cycles to execute
Again:               DEC   R16      ;-----|                      1
                     NOP            ;     |                      1
                     BRNE  Again    ;-----|                      2
;                   _______________________________________________________
;                   Total # of cycles for execution of A =   5*(1+1+2) - 1
 
 

In this code the loop is controlled by the value in R16 which is 5. So the code in block A is done 5 times. This result in C = 3 + A = 3 + [5(1+1+2) -1] = 22cycles.

One important point to note here is that the BRNE instruction takes 2 cycle to execute when it is true and 1 cycle when the condition is false. So for the 5 times the BRNE instruction is executed 1 time the condition is false which is why we subtract 1.

Step 3 - Calculate Execution Time (ET) using information from steps 1 & 2

AVR Tutorials hopes this AVR assembly tutorial on calculating execution time was beneficial to you and looks forward to your continued visits for all your microcontroller tutorial needs.