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.

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.

Real World Example

• 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

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