The P80C51 has no flash. It is not reprogrammable. It has to be used, either with the factory-programmed ROM program or with external program memory.

Mike, I don't know how many times people will have to tell you to read the datasheet. THERE IS NO FLASH in this MCU! The 80C52 has a factory programmed, non-alterable ROM.

Until the O/P understands how his emulator works, he won't be able to figure out anything about what the problem is.

When you use an 805x, meaning, in this case, a true 8051 or 8052, and not an 875x or 895x, or the like, you're dealing with a part that has no way of allowing you to provide code for it, except via external memory.

If, as I previously suggested might be the case, the emulator invokes the ONCE mode, and forces the MCU to turn off all its drivers, then the emulator-resident MCU can drive the target circuit. Unfortunately, once the emulator is gone, the MCU can't execute the code that was in the emulator, because it's not resident in the target circuit.

The O/P hasn't told us anything about his application circuit, so we don't know whether there's external memory available, in which case there's a chance something can be done with the circuit board. Otherwise, it's useless.

If the MCU is truly a P80C51 and the nEA pin is tied to Vcc, then it will never execute anything other than what's programmed, at the factory, in the MCU's internal ROM. It will never execute the program he's loaded into the emulator. It will never execute any other program he wants to run. It will only run what's in the MCU's internal ROM.

RE