Testing is a crucial part of developing any embedded system. By building testability into the system during the design phase, developers can save significant time and money over the product life cycle. This article provides an overview of key concepts and strategies for designing embedded systems with testing in mind from the very beginning.
Keys to Design for Test
Several overarching principles enable effective testing of embedded systems:
Modularity
Break the system into separate modules with clearly defined interfaces and limited dependencies between modules. This isolation allows thorough testing of each module without needing the whole system.
Observability
Incorporate features that provide visibility into the system state and operation. Test interfaces and debug capabilities allow more effective testing.
Controllability
Include capabilities to stimulate and alter internal states. This controllability enables validation of corner case behaviors.
Layered Architecture
Structure the system into layers (e.g. hardware, OS, middleware, application) with clean interfaces between layers. Testing each layer independently accelerates debug.
Simulation Models
Create simulation models of system components and environments. Simulation facilitates testing without needing physical hardware and offers controllability.
By keeping these principles in mind during design, developers can build in test features from the architecture level down to the circuit level.
Architectural Design Strategies
Several architectural design strategies help prepare an embedded system for easier, lower cost testing:
Partition into Testable Blocks
Decompose the system into testable blocks along module boundaries. Keep critical functionality isolated in test blocks with limited dependencies.
Design Test Modes & Interfaces
Define special test modes and interfaces to provide access and control of internal states for testing. Use isolation buffers to avoid test interfaces impacting system behavior.
Include Test Diagnostic Capabilities
Incorporate test diagnostic functions like self-tests, debug ports, signature analysis, and runtime log buffers to aid in test verification and fault isolation.
Simulate & Emulate Components
Provide models, mock objects, hardware emulators to simulate the test environment. This allows testing without all real components.
Follow a Test-Driven Design Approach
Take a test-first approach when designing components. Define test procedures and metrics first, then develop modules to meet testing requirements.
By applying architectural design principles focused on testability, the system can much more thoroughly be tested and potential issues identified early.
Hardware Design Considerations
In addition to software testability, embedding testing capabilities directly into the hardware design allows lower level validation:
Design Test Points for Critical Nets
Include extra test points on printed circuit boards to provide monitoring access to important internal signals during testing.
Support Scanning Methodologies
Incorporate scan chains in digital logic to stimulate and observe values deep within ICs during manufacturing test.
Partition into Testable Blocks
Group related logic into independently testable blocks. Test structures built into blocks simplify verification.
Include Functional Redundancy
Provide replicated functional blocks to enable self-checking between blocks during system operation and power-up integrity checks.
Follow Design-for-Test Methodologies
Adhere to structured Design-for-Test schemes which include test considerations in the HDL code and chip layout. Automated tools can then generate optimal test vectors.
Building in hardware test capabilities provides access to exercise and validate lower level functionality and accelerates fault grading.
Software Design Approaches
From a software perspective, designing for test entails architecting code modules and structures to support testing needs:
Modular Layered Design
Partition software into layers and modules that can be tested independently. Limit module interaction through clean APIs.
Facilitate Unit Testing
Structure software logic into functions and classes that lend themselves to modular automated unit testing at the developer workstation.
Incorporate Assertions
Embed validation checks and assertions directly into the code to continually test key assumptions and catch issues early.
Implement Test Modes
Design special software test modes that provide access to test interfaces and inject test stimuli for validation. Define entry/exit sequencing to constrain behaviors when in test mode.
Support Test Automation Frameworks
Integrate tools to automate white/black box function testing across software modules. Plan test case writer interfaces and parameters to enable test automation.
A well layered, modular software design with built-in test capabilities enables powerful test automation across software components.
Optimizing the Test Lifecycle Process
While the test design strategies above help create testable systems, organizations also need to assess and optimize their test process flows:
<table> <thead> <tr> <th>Phase</th> <th>Key Optimization Questions</th> </tr> </thead> <tbody> <tr> <td>Test Planning</td> <td> <ul> <li>Have test goals, environments, cases been defined early?</li> <li>Are test team dependencies understood?</li> </ul> </td> </tr> <tr> <td>Test Development</td> <td> <ul> <li>Are test suites modular to enable parallel test creation?</li> <li>Has automation been leveraged appropriately?</li> </ul> </td> </tr> <tr> <td>Test Execution</td> <td> <ul> <li>Can test cycles be executed unattended e.g. overnight?</li> <li>Do tests validate pass/fail clearly?</li> </ul> </td> </tr> <tr> <td>Test Result Analysis</td> <td> <ul> <li>Are regression trends understood?</li> <li>Is test grading efficient?</li> </ul> </td> </tr> </tbody> </table>By reviewing the test process using metrics and leveraging automation opportunities, the test program can achieve maximum efficiency. Resources required can be understood upfront and minimized wherever possible while still achieving test goals.
Example Designs Showcasing Test Methodologies
To illustrate how these concepts apply, below are two examples of embedded system test design architectures:
Automotive Body Controller
This system controls body electronics functionality in an automobile. The architectural design decompose into the following testable blocks:
<img src="https://docs.google.com/drawings/d/e/2PACX-1vRFEfBJM128xhyJXNuTRiGXJ46DHjTN4Z_FREDk3YFSsTzwnv0F2zjL7FAEf0pZUbNatZVOgExz4fQf/pub?w=602&h=396">Key design elements that improve test effectiveness include:
- Independent network, sensor, and actuator blocks with abstracted interfaces
- Diagnostic monitor functions and runtime exception handling
- Special manufacturing and service test modes
- CAN connectivity to vehicle networks - enables stimulus injection
- Defined APIs and middleware to simplify function testing
Microwave Radio PCB Module
This complex PCB implements specialized algorithms for signal modulation/demodulation and channel filtering in a point to point microwave radio.
Design elements to enable testing include:
- Functional partitioning into dual receiver chains and separate control logic
- Analog loopback paths for built-in-self-test
- Digital scan chains deeply embedded
- Boundary scan interface
- On-chip monitoring for out of range conditions
- Process monitors to detect process shifts
In both examples, the systems leverage modularity, diagnostics, and access interfaces to thoroughly validate functionality and accelerate defect detection.
Frequently Asked Questions
<details> <summary>What standards or methodologies help define design for test best practices?</summary>Several industry standards and structured design methodologies aimed specifically at testability provide excellent guidance for building in test features:
- IEEE 1149.1 - Defines standards for test access ports & boundary scan architecture
- IEEE 1500 – Standard for embedded core test methodology
- Design for Testability – Structured design practices focused on controllability and observability
- 6-Sigma Design – Quality methodology that emphasizes in-system monitoring & diagnostics
Adhering to standards and structured methodologies gives the best results in terms of maximizing test coverage while minimizing test cost overhead built into the system.
</details> <details> <summary>How can test requirements be captured early in the design phase?</summary>Solid test planning starts by gathering test requirements and metrics during the original system specification phase, including:
- ** Functional test cases** – The scenarios
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