First of all, he hasn't stated whether he's using external program memory, so how can you reach the conclusion that he's having the problem in question under those circumstances? Secondly, if he inserts the 89C52 where the 89C51 previously was, in the same circuit, /EA should remain unchanged. According the datasheets, this affects both MCU's in the same way.

I agree, in that, if /EA were to be unconnected, unpredictable effects would result. The /EA signal would, presumably, start out as "low", meaning "use external program memory," then, after a time, would charge up to a "high" level. On the next RESET, the signal would, being in a "high" state now, the /EA signal would tell the MCU to execute its internal code. There's no indication that that's the case, however. Further, if that were the case, he'd experience the same problem with the 89C31 as with the 89C32.

I'd guess, having no information to the contrary, the problem lies with the fact that one or both MCU's are from ATMEL.

RE