Today I was in the process of changing the connection to the TM1637 display on the 8042 board I had designed. When I powered up the board again. It failed to work. Damn, had I damaged something? I measured the voltages at various points. Power was normal, but there was no output from the DATA and CLOCK lines to the TM1637. So the 8042 wasn't booting up.

Usually this is due to poor soldering on my part so I checked the bottom of the board for any incompletely soldered pins. Then I noticed that the leads from one of the crystal load capacitors C2 and C3 in the schematic above had not been trimmed closer to the PCB and had bent to create a short across the capacitor. I must have pressed them together when I placed the PCB on the workbench. No wonder the MCU wasn't running.

That'll teach me to not leave the component leads too long when I trim them. Oh well, better to find the fault now than later when installing the PCB in the case.