X P L A N AT I O N : F P G A 1 0 1
by Adam Taylor Chartered Engineer aptaylor@theiet.org
ne of the most exciting moments in an engineering project is when the hardware arrives in the lab for the first time, ready for commissioning before integration testing. This stage in the development process typically can mean long hours and a certain amount of stress for all the engineers on the project. But tools and techniques are available to help ease the way and move the project along. Let’s take a look at how we can minimize any issues that may arise in getting a design to the next level, and how to get through the commissioning phase in a timely manner.
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THINK OF HOW YOU WILL TEST FROM DAY ONE All engineers know that the cost of implementing fixes increases as the development progresses. It is more expensive to fix a pinout error once the design has been finalized and manufactured than during an early design review, for example. This is also true for testing and integration in that the earlier you think about how you will test the hardware, FPGA, system, etc. and write the test specifications, the easier it becomes for the engineering team to include the necessary test points, hooks and functionality. The aim of the testing is to ensure you are delivering a safe system that meets the user’s specified requirements. You must therefore ensure that each of these requirements is demonstrated by test. Thus, the functional test requirements should be flowed down and traceable to the design requirements (that is, each test should show which requirements it addresses). It is also good practice to compile a design verification matrix that details how each functional requirement is to be tested—for example by test, by analysis or by read across (if the requirement was qualified or tested earlier, on another project). This docuFourth Quarter 2013
ment (Figure 1) may also show which tests are proof of design and which ones are to be used for the production run. Completing these documents early in the project life cycle will ensure that both the system design teams and the test equipment design teams will have an established baseline. However, before the functional tests can be conducted, the design engineers must first be satisfied that the underlying hardware is correct. They will typically require a hardwarelevel test specification that will include such things as voltage rails, performance and basic verification of the hardware; the latter is performed prior to the functional testing. It is worth determining the test equipment that you will need and what performance is required—for instance, do the pattern generator and logic analyzer have sufficient storage depth and operating frequency? You will also need to determine whether any more-specialized test equipment will be required, for instance arbitrary waveform generators, high-stability frequency references and the like. DESIGN PHASE INCLUSIONS During the design of the hardware, you may need to include several design features and functions to allow testing of the board with greater ease. These requirements can be simple or more in depth. The simplest and most often implemented test provision is placing test points on all voltage rails (saving the need to damage solder joints by probing on them). However, it is also good practice to place a pad connected to the ground (0V) return close to the voltage test point, to ease the testing. Protecting this test point with a high-value resistor will limit the current that could flow if it were accidentally shorted during testing. You may also wish to think about adding test pins to these pads to enable them to connect to automatic test sysXcell Journal
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