A product ( with a massive, linear power supply ) has powerfail detection - when voltage dropped below <x>, ... powerfail. This triggered a digital counter and an R/C network as an intentional race condition to trigger a line going to a PIO on the processor. Which ever finished first, won. The software would then shut things down in an orderly fashion. It would wait in that state until reboot.
When it was tested, it was tested with an A/C relay driven by the parallel port on a PeeCee. Gen a random number, wait that many milliseconds, turn the relay off ( or on ).
But field service said the thing would latch in powerfail. I pointed to the automatic (RNG driven) test of powerfail ( run every software release ) and the lead field service guy says "but that relay only switches on zero crossings of the A/C line." Sure enough... we thumbed power strips for two days and got it to latch....
The fix? Add a software counter to the powerfail state. After <n> cycles, it pulled the reset line ( which was in I/O space; NEC V25) itself.
A product ( with a massive, linear power supply ) has powerfail detection - when voltage dropped below <x>, ... powerfail. This triggered a digital counter and an R/C network as an intentional race condition to trigger a line going to a PIO on the processor. Which ever finished first, won. The software would then shut things down in an orderly fashion. It would wait in that state until reboot.
When it was tested, it was tested with an A/C relay driven by the parallel port on a PeeCee. Gen a random number, wait that many milliseconds, turn the relay off ( or on ).
But field service said the thing would latch in powerfail. I pointed to the automatic (RNG driven) test of powerfail ( run every software release ) and the lead field service guy says "but that relay only switches on zero crossings of the A/C line." Sure enough... we thumbed power strips for two days and got it to latch....
The fix? Add a software counter to the powerfail state. After <n> cycles, it pulled the reset line ( which was in I/O space; NEC V25) itself.