Interpreting time stamps in SWO trace output

Introduction

This Technical Note illustrates how to interpret timestamps in SWO trace output for IAR Embedded Workbench for ARM version 5.20, using a simple example.

Background

• A target hardware system is running with 72 MHz system clock.
• C-SPY is connected with a debug probe, using SWD.
• Exceptions are traced (SWO Setup).
• Time stamps are enabled with resolution 16.
• In the trace below, letters A,B,C,D have been added to represent the cycle values that are connected to the exception that just happened.

Edited trace example

Index   SWO Packet   Cycles       Event                     Value  
+-------+------------+------------+-------------------------+------------+
006657  0E0F20       122725344    Exit Exception Number         15
006658  20           122725376    Timestamp synch                2
006659  0E0030       122725376    Return to Exception Number     0
006660  0E0F10       122725376    Enter Exception Number        15
006661  C09323       122797360 A  Timestamp synch             4499 
006662  0E0F20       122797360    Exit Exception Number         15
006663  10           122797376 B  Timestamp synch                1
006664  0E0030       122797376    Return to Exception Number     0
006665  0E0F10       122797376    Enter Exception Number        15
006666  C09323       122869360 C  Timestamp synch             4499
006667  0E0F20       122869360    Exit Exception Number         15
006668  20           122869392 D  Timestamp synch                2
006669  0E0030       122869392    Return to Exception Number     0

Example calculations

Running exception 15

B - A = 122797376 - 122797360 = 16
16/72 ~ 0.22
The time from A to B is approximately 0.22 us.

Running main program

C - B = 122869360 - 122797376 = 71984
71984/72 ~ 999.78
The time from B to C is approximately 999.78 ms.

Running exception 15

D - C = 122869392 - 122869360 =
32/72 = 0.44
The time from C to D is approximately 0.44 us.

Comments

Since the resolution is set to 16, the real time spent in the 2 exceptions is somewhere between 0.22 and 0.44 us.

To measure it more exactly, you could use timestamp resolution 1. In the example, it seems that exception 15 is a 1 ms periodic interrupt. (Note that the Interrupt Log, Interrupt Log Summary and Timeline windows can be used to see time in us/ms format).

All product names are trademarks or registered trademarks of their respective owners.