Thanks Babar and Peter,

I should give more details about my project requirements.

I need atleast 4KB internal ROM OTP or MTP flash.
I have implemented a software RTC, so I can put the MPU to idle mode not in powerdown mode, otherwise oscilator will be stopped updating timer and RTC would not work, am I right?

I might use 89C4051, but it is not available in our area. I would rather prefer to have a full fledge 8051 with more i/o lines.

I used 89C2051 for the same project but because of flash (2K) limitations I have to migrate to Atmel 89C52.
Currently I am using 89C52 but it is drawing 25mA+ in operation and over 10mA in idle mode. And it works at minimum 4 volts to 6 volts.

I am developing my application with 89C52 because it is MTP. After I finalized my code I might use some cheaper (price & power both) OTP.

I would appreciate if some one can recommend me even low power OTP.
keeping in mind that I am handicapped not to put cpu in power down mode. The maximum my code can afford is Idle mode.

Thanks

Rauf