Safety Architecture Patterns

/* Monitor pattern: main + monitor channel for speed signal */

#include "SpeedMonitor.h"
#include "SafetyManager.h"

#define SPEED_PLAUSIBILITY_THRESHOLD_KMH  5.0F  /* max allowable delta */
#define SPEED_FAULT_COUNT_LIMIT            3

static uint8_t fault_count = 0u;

/* Called every 10ms from safety monitor task (ASIL-B) */
void SpeedMonitor_Step(float32_t main_speed_kmh,
                       float32_t monitor_speed_kmh) {
    float32_t delta;

    /* Compute absolute difference between channels */
    delta = main_speed_kmh - monitor_speed_kmh;
    if (delta < 0.0F) { delta = -delta; }  /* abs() without math.h */

    if (delta > SPEED_PLAUSIBILITY_THRESHOLD_KMH) {
        fault_count++;
        if (fault_count >= SPEED_FAULT_COUNT_LIMIT) {
            /* Three consecutive discrepancies: declare fault */
            SafetyManager_SetFault(FAULT_SPEED_SENSOR_DISCREPANCY);
            /* Safe state: reduce maximum speed demand */
        }
    } else {
        fault_count = 0u;  /* Reset on consistent readings */
    }
}

🔬 Deep Dive — Core Concepts Expanded

This section builds on the foundational concepts covered above with additional technical depth, edge cases, and configuration nuances that separate competent engineers from experts. When working on production ECU projects, the details covered here are the ones most commonly responsible for integration delays and late-phase defects.

Key principles to reinforce:

• Configuration over coding: In AUTOSAR and automotive middleware environments, correctness is largely determined by ARXML configuration, not application code. A correctly implemented algorithm can produce wrong results due to a single misconfigured parameter.
• Traceability as a first-class concern: Every configuration decision should be traceable to a requirement, safety goal, or architecture decision. Undocumented configuration choices are a common source of regression defects when ECUs are updated.
• Cross-module dependencies: In tightly integrated automotive software stacks, changing one module's configuration often requires corresponding updates in dependent modules. Always perform a dependency impact analysis before submitting configuration changes.