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Real-Time
========= 

Real time is being able to respond to an interrupt in a bounded maximum time.

An analysis by OMAC User Group showed that 95% of real-time applications require between 0.5ms to 10 ms respond time and could tolerate 10% variations, or 50µs to 1ms of jitter.

.. figure:: Jitter.png

    Typical real-time requirements.

The important thing is to be able to respond with a sufficient amount of time to handle 
the interrupt before exceeding the system's required tolerances. And to be able to do
so consistently. Always responding within a bounded amount of time.

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Real World Example
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* A Microsoft partner wanted to know if Windows CE was 'HARD' real-time

  * Wanted to know if CE was capable of running GSM radio and UI at the same with one CPU.
  * They were concerned that CE was not HARD real-time enough to meet the requirements

* The requirements

  * Run GSM cellular radio DSP
  * Meet "tight" timing requirements   
  * Target CPU ARM9 250Mhz
  * Full Windows CE UI
  * And play video


Real World Timing Requirements
==============================
* So what where the actually requirements?
* Interrupt every 4.6 ms
* Allowable jitter of 0.5us

.. figure:: Response_Requirements.png

    Required timing constraints for cellphone project

Windows CE Test Results
=======================
We did a quick test using the following configuration:

* Samsung SMDK2410 development board
* 200 MHz ARM with 16x16 cache   
* Windows CE 5.0 with full UI
* Running a WMV video

Here's what we got:

========    ===========     ===========
Value        ISR starts      IST starts
========    ===========     ===========
Minimum      1.2             31.7
Average      3.3             67.2
Maximum      13.3            103.0
========    ===========     ===========

Result
------
* In terms of the 0.5 ms Jitter alone

  * Longest ISR response time was 13.3 μs or 2.6% of max jitter allowed
  * Longest IST response time was 103 μs or 20.6% of max jitter allowed

* Conclusion

  * Response time was well within the requirements.
  * The project was approved and went ahead.

* Lesson

  * What's important is "Sufficient" time