AN3129

IRQ latency measurement

Doc ID 16927 Rev 1

7/12

8, 35

9, 29

10, 13

11, 0

……

……

198, 0

199, 0

200, 0

3.

To stop statistics capture, use one of the following commands (depending on the

mode):

–

#echo 0 > irq_mode

or

–

Once the mode stopped, the timer is stopped and the IRQ handling is deactivated. Also the

statistics are cleared.

4.2

Module internals

The module uses a general purpose hardware timer to raise an IRQ/FIQ and capture the

ticks elapsed. The timer automatically reloads itself after raising an interrupt. In this way

when the IRQ handler reads the timer value, this value is the exact measurement of the

system latency for handling this IRQ.

Depending upon the choice of IRQ handling mechanism (IRQ or FIQ), the corresponding

selection is made in VIC handling. FIQ registration and de-registration is part of the

standard Linux kernel.

Timer (GPT) interval: 1 millisecond

Timer tick interval: 1/48 microsecond

Interrupt handler latency frequency counter is captured from 0 to 9600 ticks. This

corresponds to a range of 0 to 200 microseconds. This frequency range is sufficient for

determining the interrupt latency performance.

4.3

Performance measurements

The details of setup used for making the measurements are:

●

Kernel: Linux kernel version 2.6.27

●

Board: SPEAr Plus 600 application board

●

CPU: ARM core running @333 MHz

●

RAM: 128 MB DDR2 running @333 MHz

●

Flash: 8 MB NOR