Documentation, Design for and Writing Factory Test #7
Description
AI Overview
Writing a factory test for an embedded system involves a two-pronged approach: designing the hardware for testability and developing specific test firmware/software. The goal is to quickly and automatically verify the proper function of every unit coming off the production line.
Design for Testability (DFT)
Incorporate testing considerations early in the design phase:
Add test points and headers: Include physical access points (e.g., test pads, pin headers) to key pins and power circuits for probing and monitoring with external equipment like JTAG debuggers, oscilloscopes, or automated test equipment (ATE).
Include a serial interface: A basic serial connection (like UART to USB) is essential for monitoring the application's status, capturing data, and sending commands from a host computer during testing.
Modularize hardware and software: Separate hardware-dependent code (drivers) from core logic using abstraction layers to enable easier testing, including the use of mocks/stubs if necessary.
Design a "device under test" mode: Integrate a specific mode into your production firmware, or a dedicated test firmware, that allows the main microcontroller to be controlled by an external test system.
Factory Test Implementation
The manufacturing test process typically involves two main stages: board-level testing and assembled device testing.
Board-Level Testing
In-Circuit Test (ICT): This is typically the first step, often using a "bed-of-nails" fixture to check for manufacturing defects such as shorts, opens, and incorrect component placement. It measures basic electrical parameters like resistance, capacitance, and voltage levels for individual components.
Initial Programming/Provisioning: The manufacturing firmware is flashed onto the microcontroller, and basic information like a serial number is provisioned at this stage.
Functional Testing (FCT)
This verifies that the assembled product's functionality meets design specifications under power.
Develop a Test Plan: Define clear test scenarios, expected outputs, and pass/fail criteria based on product requirements.
Write Test Software (Firmware/Host):
On-device firmware: Program the device with a specific test application or enable a test mode within the main firmware. This code will execute test routines and report results.
Host automation software: Create a PC-based application (e.g., using Python, LabVIEW, C#) that communicates with the device under test via the serial interface or debug port. This application controls the test sequence, sends commands, and logs results.
Automate the Process:
Setup: The host software connects to the device, checks initial conditions (e.g., power mode, OS version), and puts it in the test mode.
Execution & Verification: The host sends commands to the device, which performs a specific action (e.g., turn on LED, read sensor, enable communication interface). The host or an external sensor/camera then verifies the expected output or response.
Logging & Teardown: Log detailed results (pass/fail, specific measurements, error messages). The system should be able to reset or clean up its state to be ready for the next unit.
Focus on Specific, Simple Tests: Each test case should be simple and test one thing at a time (e.g., check LED color, verify communication protocol) to make debugging failures easy.
Key Best Practices
Automate everything feasible to ensure consistency and speed up the production line.
Implement clear error handling and logging within the device firmware and the host software to quickly identify the root cause of a failure.
Use robust test fixtures (like bed-of-nails or custom test heads) that provide reliable, repeatable connections to the device.
Maintain traceability between your product requirements and your test cases to ensure comprehensive coverage.