Architecture Decision Records - System Architecture

What Is an Architecture Decision Record?

ADR Definition

An Architecture Decision Record (ADR) is a short document that captures a single significant architectural decision, its context, the options considered, the decision made, and the rationale. ADRs are stored alongside the architecture artefacts (in a git repository or architecture tool) and form the institutional memory of why the architecture looks the way it does.

Without ADRs, new team members and future architects reverse-engineer past decisions incorrectly, re-open settled debates, or unknowingly violate constraints that exist for good reasons. With ADRs, every architectural constraint is explained and the trade-offs are visible.

ADR Template for Automotive Projects

MarkdownADR-007-zone-ecu-mcu-selection.md

# ADR-007: Zone ECU MCU Family Selection

## Status
Accepted (2024-11-12)

## Context
Zone ECUs require: ASIL-D capability, dual-core lockstep for safety monitor,
CAN-FD + 100BASE-T1 Ethernet integrated, HSM for secure boot and OTA,
operating temperature -40 to +125 °C, 10-year supply guarantee.

## Decision
NXP S32K3 family (S32K344 for Front zone, S32K324 for Rear zone).

## Options Considered
| Option | ASIL-D | CAN+Eth Integrated | Supply | Cost Index |
|--------|--------|--------------------|--------|------------|
| NXP S32K3 | Yes (lockstep) | Yes (native) | 10+ yr | 1.0 |
| Renesas RH850/U2A | Yes | Partial (ext PHY) | 10+ yr | 1.1 |
| Infineon TC397 | Yes | No (ext Eth) | 10+ yr | 1.3 |
| STM32H7 | No (ASIL-B only) | No | 5 yr | 0.7 |

## Rationale
S32K3 selected for: native 100BASE-T1 eliminates external PHY BOM cost ($2.40/ECU),
dual-core lockstep meets ASIL-D without external monitor IC,
S32K344 HSM satisfies EVITA Full for OTA signing.
STM32H7 rejected: ASIL-B ceiling insufficient for chassis functions.

## Consequences
+ Eliminates external Ethernet PHY: −$2.40/ECU × 3 zones = −$7.20/vehicle
+ Single toolchain (NXP CodeWarrior / S32 Design Studio) across zones
− Renesas familiarity in existing team requires training (estimated 20 person-days)
− S32K344 LQFP-176 package requires new PCB layout rules

## Review Date
2025-11-12 (annual review or when NXP S32K4 roadmap confirmed)

Summary

ADRs transform architecture governance from a bureaucratic approval process into a living knowledge base. The most valuable ADRs are not the ones documenting decisions that everyone agreed on immediately -- it is the ones documenting decisions where multiple reasonable options existed and trade-offs were genuinely difficult. An ADR for the MCU selection that shows the STM32H7 was seriously considered and rejected for a specific technical reason (ASIL-B ceiling) prevents that debate from being re-opened by every new engineer who joins the project and is familiar with ST microcontrollers. The "consequences" section is the most important part: it forces architects to explicitly acknowledge what was given up, creating accountability for the trade-offs.

🔬 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.

🏭 How This Topic Appears in Production Projects

Project integration phase: The concepts covered in this lesson are most commonly encountered during ECU integration testing — when multiple software components from different teams are combined for the first time. Issues that were invisible in unit tests frequently surface at this stage.
Supplier/OEM interface: This is a topic that frequently appears in technical discussions between Tier-1 ECU suppliers and OEM system integrators. Engineers who can speak fluently about these details earn credibility and are often brought into critical design review meetings.
Automotive tool ecosystem: Vector CANoe/CANalyzer, dSPACE tools, and ETAS INCA are the standard tools used to validate and measure the correct behaviour of the systems described in this lesson. Familiarity with these tools alongside the conceptual knowledge dramatically accelerates debugging in real projects.

⚠️ Common Mistakes and How to Avoid Them

Assuming default configuration is correct: Automotive software tools ship with default configurations that are designed to compile and link, not to meet project-specific requirements. Every configuration parameter needs to be consciously set. 'It compiled' is not the same as 'it is correctly configured'.
Skipping documentation of configuration rationale: In a 3-year ECU project with team turnover, undocumented configuration choices become tribal knowledge that disappears when engineers leave. Document why a parameter is set to a specific value, not just what it is set to.
Testing only the happy path: Automotive ECUs must behave correctly under fault conditions, voltage variations, and communication errors. Always test the error handling paths as rigorously as the nominal operation. Many production escapes originate in untested error branches.
Version mismatches between teams: In a multi-team project, the BSW team, SWC team, and system integration team may use different versions of the same ARXML file. Version management of all ARXML files in a shared repository is mandatory, not optional.

📊 Industry Note

Engineers who master both the theoretical concepts and the practical toolchain skills covered in this course are among the most sought-after professionals in the automotive software industry. The combination of AUTOSAR standards knowledge, safety engineering understanding, and hands-on configuration experience commands premium salaries at OEMs and Tier-1 suppliers globally.