I use 89C51ED2 (44 pin PLCC) in my circuit and I am facing a peculiar problem. The reset circuit of micro-controller consists of 8.2K and 10uF capacitor. When powered on the reset pin (PIN 10) remains in HIGH state, even though it is pulled down to ground by 8.2K resistor. I checked that RESET pin is not shorted to VCC. This problem persists only in a few PCBs and rest of the PCBs works fine. I have used identical components in all PCBs. But when I remove the micro controller, PIN 10 of IC base comes to LOW state. What can be the reason for this problem? Thanks in advance.
Defective or counterfeit CPU?
If you plug in a known good CPU, does that still work? Or does it get damaged?
If it works, then the one that doesn't is defective or damaged.
If it damages the good CPU, review your PCB for shorts or other flaws, or incorrectly placed/selected components.
CPU is not damaged when placed in problematic PCB. The same CPU works in another PCB without any problem. Issue may with the reset circuit of PCB. But PCBs are thoroughly tested, the tracks are correct and components used are identical for all PCBs.