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Cycle Budget Calculator

Run the Cycle Budget Calculator MicroSim Fullscreen

You can include this MicroSim on your website using the following iframe:

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<iframe src="https://dmccreary.github.io/fft-benchmarking/sims/cycle-budget-calculator/main.html"
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About This MicroSim

Every real-time DSP task comes with a hard question: how many CPU cycles do I actually get? The answer is a simple product:

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cycle budget = clock speed (Hz) x deadline (seconds)

At the Pico 2's stock 150 MHz with a 40 ms deadline, that is 6,000,000 cycles. That number is your entire allowance. If an FFT costs more than that, it does not matter how elegant the code is — the deadline is missed.

The sim opens deliberately over budget: the sample workload is set to 8,000,000 cycles against a 6,000,000-cycle budget, so the capacity bar is fully red and the readout says OVER BUDGET by 2,000,000 cycles. Your job is to find the combinations of clock speed and deadline that bring it back into range.

Each number in the equation is drawn in the color of the slider that controls it, so you can see at a glance which input you are changing.

How to Use

  1. Read the opening state. Why is the bar entirely red?
  2. Predict, before you move anything: if you double the deadline from 40 ms to 80 ms, does the workload fit? Now drag Deadline and check.
  3. Return the deadline to 40 ms. Now find the minimum Clock speed that fits the 8,000,000-cycle workload. (The answer is 200 MHz.)
  4. Set the workload to 3,000,000 cycles and read the headroom percentage. What does headroom buy you in a real system?
  5. Check Show formula whenever you want the general relationship with units displayed alongside the worked numbers.

Controls

Control Range Default Purpose
Clock speed 1-200 MHz 150 MHz The Pico 2's stock speed is 150 MHz
Deadline 5-100 ms 40 ms How long the task has to finish
Workload 100,000-20,000,000 cycles 8,000,000 Cost of the sample task
Show formula — off Reveals the general equation with units

Lesson Plan

Grade Level

Undergraduate (college junior/senior)

Duration

10-15 minutes

Prerequisites

  • A CPU executes a fixed number of clock cycles per second
  • Basic unit conversion between milliseconds and seconds

Learning Objective

Students will be able to calculate the CPU-cycle budget available for a real-time task given a clock speed and a deadline, and apply that budget to judge whether a given workload fits.

Activities

  1. Predict-then-check (5 min): For three parameter pairs supplied by the instructor, students compute the budget by hand before setting the sliders.
  2. Find the boundary (5 min): Students find the exact clock speed at which the default workload just fits, then explain why headroom of zero is a risky place to ship.
  3. Trade-off discussion (5 min): Raising the clock costs power and heat; loosening the deadline costs responsiveness. Which would you spend first?

Assessment

Ask students to compute the budget for 133 MHz with a 25 ms deadline (answer: 3,325,000 cycles) and state whether a 4,000,000-cycle workload fits.

References

  1. RP2350 Datasheet — clock architecture and maximum rated frequencies for the Pico 2.
  2. Rate-monotonic scheduling — the formal treatment of deadlines and CPU utilization budgets.