Scheduler Options

dsPIC30F dsPIC33F dsPIC33E dsPIC33C dsPIC33A PIC32

☀️ Light

The Scheduler Options block configures the behavior of the built-in Rate Monotonic Scheduler used for multitasking in MCHP Blockset models. The scheduler manages execution of multiple tasks at different sample rates with preemptive priority-based scheduling. Rate Monotonic Scheduling Principle: - Highest rate = Highest priority - Faster tasks automatically get higher priority - Preemptive execution - Higher priority tasks interrupt lower priority tasks - Deterministic behavior - Guaranteed timing if schedulability conditions are met - Automatic priority assignment - No manual priority configuration needed

When to use:

When to use:

• Model has multiple sample times (fast current loop + slow position loop)
• Need to control task overload behavior (queue, skip, or delay)
• Timing jitter observed — change from Queue to Skip mode
• Critical task must never be skipped — use Delay mode
• Tasks occasionally overrun — configure graceful handling

When NOT to use:

• Single-rate model with one sample time — default scheduler suffices
• All tasks complete well within their period (no overload risk)
• Default overload behavior (Queue) works for your application

Parameters

Task Overload Behavior

Overload Behavior Comparison

// Task with 10 ms period, execution takes 12 ms (overload)

Option 1: Queue (queue length = 1)
t=0ms:   Task starts
t=10ms:  Trigger → QUEUED (1 pending)
t=12ms:  Task ends → immediately starts queued execution
t=20ms:  Trigger → LOST (queue full)
t=24ms:  Task ends
t=30ms:  Normal execution resumes

Option 2: Skip once
t=0ms:   Task starts
t=10ms:  Trigger → SKIPPED
t=12ms:  Task ends
t=20ms:  Normal execution
t=30ms:  Normal execution

Option 3: Delay all
t=0ms:   Task starts
t=10ms:  Trigger → DELAYED
t=12ms:  Task ends → delayed trigger executes immediately
t=24ms:  Task ends (ran at t=12)
t=30ms:  Back to normal (with 2ms delay accumulated)

Rate Monotonic Scheduler Architecture

Task Priority Assignment

Multitasking Configuration

Simulink Configuration Requirements

• Solver Type: Fixed-step (required for code generation)

Configuration Parameters → Solver
Type: Fixed-step
Solver: discrete (no continuous states)

• Sample Times: Define task rates as integer multiples

Base rate: 50e-6     % 50 µs (20 kHz)
Task 1:    100e-6    % 2× base rate
Task 2:    500e-6    % 10× base rate
Task 3:    5e-3      % 100× base rate

• Tasking Mode: Enable multitasking in configuration

Configuration Parameters → Code Generation → Interface
Multi-instance code: off
Single output/update function: off
⚠️ Let MCHP blockset manage tasking automatically

Scheduling Algorithm

// Simplified scheduler pseudocode

        execute_task(highest);
        mark_complete(highest);
        ready_tasks = check_triggered_tasks();
    }
}

Schedulability Analysis

Liu & Layland Utilization Bound

For Rate Monotonic Scheduling, the system is schedulable if:

U = Σ(Cᵢ/Tᵢ) ≤ n(2^(1/n) - 1)

Where:
  U  = Total CPU utilization
  Cᵢ = Execution time of task i
  Tᵢ = Period of task i
  n  = Number of tasks

For n tasks:
  n=1: U ≤ 100%
  n=2: U ≤ 82.8%
  n=3: U ≤ 78.0%
  n→∞: U ≤ 69.3%

Example Calculation

Task 1: C₁=20µs, T₁=50µs   → U₁ = 20/50 = 0.40 (40%)
Task 2: C₂=80µs, T₂=500µs  → U₂ = 80/500 = 0.16 (16%)
Task 3: C₃=1ms,  T₃=10ms   → U₃ = 1/10 = 0.10 (10%)

Total: U = 0.66 (66%)
Bound for n=3: 0.78 (78%)

✓ System is schedulable (66% < 78%)

Advanced Topics

Interrupt-Driven Tasks

Tasks triggered by hardware interrupts (using MCHP_Interrupt block) have configurable priorities independent of sample time:

// Interrupt priority can override rate monotonic assignment
Interrupt priority: 1-7 (device-dependent)
Multiplied by -100 for scheduler priority

Example: Priority 7 interrupt → scheduler priority -700
         (higher than any rate monotonic task)

Context Switching Overhead

Examples

Programmatic Setup

% Add block to model
add_block('MCHP_Blockset/System Configuration/Scheduler Options', [mdl '/Scheduler']);

% Configure key parameters
set_param([mdl '/Scheduler'], 'TaskOverloadBehaviour', 'Queue the task (queue length is 1)');

Example 1: Motor Control Application

% Three-level motor control hierarchy
% Base rate: 50 µs, Overload: Queue

Sample times:
- Current loop:    50e-6    (Priority 40 - highest)
- Speed loop:      500e-6   (Priority 30)
- Position loop:   5e-3     (Priority 20)
- Communication:   100e-3   (Priority 10 - lowest)

Scheduler Options:
TaskOverloadBehaviour: 'Queue the task (queue length is 1)'

% Result: Current loop can preempt all others
% Brief overloads are queued and caught up

Example 2: Data Acquisition System

% Robust data logging with guaranteed timing
% Base rate: 100 µs, Overload: Delay all

Sample times:
- Fast ADC:       100e-6   (Priority 40)
- Slow sensors:   10e-3    (Priority 30)
- Data logging:   100e-3   (Priority 20)
- Display update: 500e-3   (Priority 10)

Scheduler Options:
TaskOverloadBehaviour: 'Delay all new tasks until end of overload'

% Result: No samples lost, timing may shift temporarily

Example 3: Communication System

% Non-critical monitoring allows missed samples
% Base rate: 1 ms, Overload: Skip

Sample times:
- Protocol handler: 1e-3   (Priority 40)
- Data processing:  10e-3  (Priority 30)
- Status update:    100e-3 (Priority 20)

Scheduler Options:
TaskOverloadBehaviour: 'Skip this task execution once'

% Result: Skipped status updates acceptable

Troubleshooting

Related Blocks

• MCHP_Master - Main configuration block
• MCHP_Interrupt - Interrupt-driven task configuration
• MCHP_MCU_LOAD - CPU utilization monitoring
• MCHP_MCU_OVERLOAD - Overload detection
• MCHP_IdleTask - Background task execution