The system could just barely change all those voltages with the chip itself installed. With the very slight additional capacitive pin loading of an in-circuit emulator, it couldn’t. With a few minor hardware hacks to the target board, we were able to attach the processor and memory Vcc lines to a bench-top supply and make the problem go away -- or cause it to come back by setting the current limit of the supply right past the hairy edge of being overloaded.
We presented this finding to the customer, who then admitted that a much earlier design team had put together the switching supply for use on a much earlier product in the family. Later teams had just “copied that page from the earlier schematic, since the design worked.” Several generations of designers had skipped the process of checking the power budget, and several generations of new products had continued to add new features, loading the switching converter further every time.
The customer then realized, “This could have something to do with a few previously inexplicable field failures of earlier models.” In fact, when he went back to recheck power budgets on those designs, he found that they too had been exceeded, and a recall was in order for production units that had found their way through sales channels and into the field.
The takeaway: Beware of cutting and pasting things from earlier designs -- they may not fit. And always check your power budget.
This entry was submitted by Eric Overton and edited by Rob Spiegel.
Eric Overton is the CEO of Focus Embedded, an Austin, Texas, company dedicated to designing embedded control and instrumentation systems. His 25 years of experience spans the design of laser measurement systems, data communications test equipment, feedback control systems, microprocessor emulators, and development and video systems. He received his undergraduate and graduate degrees from the Thayer School of Engineering at Dartmouth College.
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