A few years in the past I labored within the R&D division of an organization which developed and manufactured specialised client merchandise. Certainly one of our merchandise had a separate energy provide which included an LCD show and programmable controls. These have been the times of 8-bit microcontrollers and monochrome LCD shows.
I used to be tasked with designing a take a look at jig for the principle board of the ability provide. The take a look at setup I designed was primarily based round a Nationwide Devices knowledge acquisition system, with some specialised management software program working on a PC, and an in-circuit programmer which programmed the board with the ultimate software program model on the finish of the take a look at cycle. Connection to the board was with a operated by hand ICT (in circuit take a look at) jig with pogo pins contacting take a look at factors on the underside of the board.
Operation of the take a look at setup was quite simple. The operator would place the board on the jig, decrease the highest plate, scan the serial quantity and press “take a look at.” For a superb board the outcome “take a look at handed” would seem on the display screen, in any other case a message with the character of the board fault can be proven.
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The jig was constructed by a contractor, I constructed the interface board, wrote the take a look at software program, and did the debugging. All the things went nicely. The boards have been examined and programmed, and the tester recognized board faults. So, off it went to the manufacturing flooring.
Bother began instantly. The tester was flagging the boards nearly as good, however when assembled within the energy provides, the LCD screens have been displaying random characters and basic gibberish. A number of the energy provides wouldn’t function correctly. After all, all of it got here straight again to my desk with a requirement for an evidence.
Discovering the trigger wasn’t tough, really, and concerned examination of the reminiscence contents.
When testing the jig, I had been working at my very own tempo. However on the manufacturing flooring, the technicians labored a lot sooner. As quickly because the display screen flashed the “go” message, they’d carry the board off the jig inside a number of seconds. This launched the “reset” pin of the microprocessor, and the capacitors on the board nonetheless held sufficient cost to energy up the circuit, so the processor would instantly begin to run its program for the very first time.
This system would go to the flash reminiscence to retrieve the working parameters, and, discovering the reminiscence empty, would begin to write default parameters. At this level, the capacitors would run out of cost, and the processor would energy off in the course of a flash reminiscence write cycle, leaving the reminiscence with undefined contents. When powered up after meeting, the processor would discover parameters within the reminiscence, not notice that they have been corrupted, and put them up on the display screen.
The right repair would have been to re-write the operational software program so as to add extra reminiscence checks, however time was quick and the software program folks overworked and in a foul temper, so I merely added a 2-second delay between powering off the take a look at jig and displaying the “take a look at handed” message. After that, it was all plain crusing.
Benny Attar presently works as engineering group chief in an EMC take a look at and certification laboratory.
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