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Testing as an art (Part 1)

Posted: 17 Apr 2014  Print Version  Bookmark and Share

Keywords:MCUs  testing  test equipment design  PCB  emulators 

Manufactured product, whatever it may be, should be subject to testing. Certainly, any product that I have designed that is going to be produced has a test procedure associated with it. The great advantage of testing products based on microcomputers (MCUs) is that they can be used to speed up, or even execute, the whole test.

I work for a small organisation. As a result, I get involved in all aspects of a product, including the design of the production test. Often, the test equipment design is orders of magnitude more complex than the equipment it is testing and—generally speaking—I have more fun designing the test than the product itself. Having been with the same company (and nominally in the same position) for 20 years, I have seen many different approaches used. Although it is still evolving, I feel I have some insight into the process. My interpretation of testing includes calibration and downloading the program into the MCU.

Downloading the firmware
I know of no modern microcomputer or flash memory that cannot be programmed in-circuit. Although all MCUs have some means to connect a programmer to the product, or use some kind of bootloader, I have discovered that our subcontractors actually prefer to program the MCU before PCB insertion. This is because, if there is a problem with the MCU, they can return it.

One problem that "rides beneath the radar" is that of the connection to the board. You may be surprised to learn that the guaranteed number of insertions for a connector can be in the tens. I know I have tried flat cable connectors for emulators/JTAG etc. many hundreds of times; for production, however, perhaps you should consider spring-loaded contacts like those used on a bed-of-nails (maybe integrate the programming header into the bed of nails). While at a previous Design West conference, I discovered a company called Tag-Connect, which makes a header of these contacts on a flying lead.

Figure 1: The Rabbit module is programmed via a flat ribbon cable. We made a bed-of-nails to program it, allowing the subcontractor to return the module if it fails, and saving wear-and-tear on (and subsequent replacement of the flat cable.

Product or subassembly
The device should be tested as close to completion as possible, but there are provisos. If there is some process to be performed before complete assembly, then obviously this should be undertaken as a subassembly, but it may require further testing when completed.

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