The RD2 is a remarkable family - while the '51 is undoubtly the more widely second-sourced mcu, the RD2 is a sub-family with more than one second-source! Except Philips and Atmel, RD2s are also manufactured by Windbond and SST (and according to their website also Macronix, but that I think is really exotic, not pin compatible, and 3V only...)

The differences between Atmel and Philips 89C51RD2 are mainly in peripherals and the IAP procedures, although the compatibility level is high - for details you need to compare the datasheets. Note also that the Atmel RD2 is not available in DIP40, but the ED2 is, and it is the same as RD2 just with added EEPROM.

Note also that Philips has a different type of RD2, the P89V51RD2, which is maybe less compatible with the P89C51RD2 than the Atmels' - again with regards of peripherals, but mainly the IAP.

Jan Waclawek