Muthu:

Your use of the external address bus to interface to the selects and addresses of the ADC0808 appears to be confusing. Looking at the typical application diagram of the data sheet:



It would seem to me that the two places where you are using the MOVX instructions in your code that you are not setting the DPTR to a value of 16-bit address that will end up selecting the external address that you wanted. The circuit diagram also shows that some address decode must be supplied to the READ & WRITE gate logic. How do you actually have this connected.

Please check the usage of the MOVX carefully. Use an oscilloscope to check the external address bus as your loop runs to see if what you are generating matches the circuit requirements.

Michael Karas