Calculator Inputs
Example Data Table
| Task | Exec (µs) | Period (ms) | Deadline (ms) | Ctx/Job | Expected Util % |
|---|---|---|---|---|---|
| Control Loop | 220 | 5 | 5 | 2 | ≈ 4.40 |
| Sensor Read | 180 | 10 | 10 | 2 | ≈ 1.80 |
| Comms | 350 | 20 | 20 | 2 | ≈ 1.75 |
Formula Used
- U_RMS_bound = n(2^(1/n) − 1) for n periodic tasks with D=T (tasks-only).
- EDF quick check: total load ≤ 100% (under ideal assumptions).
How to Use This Calculator
- List each periodic or sporadic task with execution time and minimum period.
- Keep units consistent: execution in microseconds, period and deadline in milliseconds.
- Enter tick rate and tick overhead if your system uses a periodic tick.
- Enter context switch time and estimated switches per job for each task.
- Add an interrupt load estimate for DMA, drivers, and fast ISRs.
- Press Submit to see totals above the form and compare checks.
- Use the CSV/PDF buttons after a calculation to export a report.
Utilization planning for deterministic deadlines
Real-time designs fail quietly when CPU margin disappears. This calculator converts task timing into a single utilization budget you can compare against a safe operating target. For example, a task with C=200 µs and T=5 ms contributes 4.00% load. If three similar tasks run, tasks-only utilization becomes 12.00% before overheads and interrupts. Many engineers keep utilization below 80% to limit response-time variance during bursts, startup phases, and flash writes.
Capturing task execution times that reflect reality
Use measured worst-case execution time, not averages, especially on cached systems. Take multiple runs, keep the highest value, and include driver calls in the same measurement window. A control loop that ranges 160–220 µs should be entered as 220 µs. If your period is 2 ms, that single task already consumes 11.00% of CPU time. Record the compiler flags, clock speed, and memory state, because these change WCET more than you expect.
Accounting for tick, context switching, and interrupt work
Overheads are small per event but significant at high rates. With a 1,000 Hz tick and 3 µs tick work, tick utilization is 0.30%. Context switching scales per job: at 2 µs per switch and 2 switches per job, a 5 ms task adds 0.08% overhead. Interrupt load can be modeled as a fixed percentage when precise counting is difficult. Tickless modes reduce this term near idle greatly.
Reading RMS and EDF indicators with engineering caution
The RMS bound shown is conservative for periodic tasks when D=T. Passing the bound suggests a comfortable region, while exceeding it is not an immediate failure. EDF’s quick check is simpler: total load at or below 100% is necessary under ideal assumptions. In practice, aim for 70–85% total load to absorb jitter, cache misses, and bursty ISRs.
Improving headroom and producing sharable reports
When load is high, first reduce C by optimizing hot paths, fixed-point math, and DMA usage. Next, increase T where latency permits, or merge periodic tasks to reduce context switches. Lowering tick rate can also reduce tick utilization if your system allows tickless idle. After tuning, export CSV for reviews or PDF for sign-off, preserving inputs, totals, and per-task contributions.
FAQs
1) What does “Total CPU Load” include?
Total CPU Load sums task utilization plus optional tick, context switching, and interrupt load. If overheads are unchecked, the result reflects tasks-only utilization for early sizing.
2) Why can a system miss deadlines below 100% load?
Jitter, priority inversion, cache effects, and clustered interrupts can delay ready tasks even when average utilization is low. Keep headroom and validate with traces on target hardware.
3) How should I choose “context switches per job”?
Start with 2 for a typical ready-to-run cycle (switch in and switch out). Increase it if your task blocks frequently on queues, semaphores, or timeouts.
4) Should I enter deadlines different from periods?
If your task must finish sooner than its period, enter a smaller deadline for documentation. The utilization math uses the period; use the deadline field to capture constraints for reviews.
5) How do I estimate interrupt load accurately?
Measure ISR time with cycle counters or trace tools, then divide total ISR time per second by one second. If tools are limited, begin with 1–5% and refine after profiling.
6) What target headroom is practical for embedded controllers?
Many teams target 15–30% headroom for stable latency under bursts. Safety-critical systems often reserve more, especially with networking, storage, or complex sensor stacks.