Beware of the Borrowed Design

It was on this exact address where the processor made the final jump from upper memory to lower memory (just after switching processing modes) that the target board with emulator attached lost its mind and jumped to a totally random address to begin executing garbage instructions. Because the problem occurred so consistently on that one jump instruction (and didn’t occur with an actual processor chip installed), the customer was convinced the emulator was at fault.

We had debugged plenty of code that made the real-to-protected mode transition with no trouble, so we were doubtful of a fault with the emulator. Yet the customer’s early setup code for the transition looked correct, so we were hard pressed to blame his code and tell him the problem was somehow in that portion of his code -- particularly since it did run on the actual processor chip itself.

Finally, in desperation, we asked the customer to send us his target system so we could replicate the problem -- which we did quite easily. But we noticed one thing the customer didn’t: The switching power supply on his board (which drove the whole processor and memory subsystem, as well as several peripheral devices) seemed awfully small for the number of chips on the board. It was so small, in fact, that we went back and redid a rough calculation of his power budget and found the switching supply to be nominally about 40 percent undersized.

We then took a hard look at the power rails on the board and found that they sagged just enough momentarily to cause the processor to run at below its minimum spec for Vcc with the emulator installed. Putting the processor in where the emulator was, we saw a similar sag, only not quite as pronounced. Ground also tended to “bounce” noticeably in both cases, but less so with the actual processor than it did with the emulator.

Working back, we cross-triggered a logic analyzer and an oscilloscope and discovered the point of “power sag” was on the second major jump instruction -- exactly where the code went wild when running under emulation.

The conclusion? On that second jump instruction, the switching supply, which was already huffing and puffing, had the job of changing the value on just about every single one of its address lines from “high” to “low” all at once. That’s a lot of signals simultaneously going from Vcc to ground.