Hello Mr. Stanley Lio,

Atmel has an application which will perform Analog to Digital (ADC) conversion using the 89C2051 integrated circuit which is plentiful and cheap around the entire world. I have built this circuit and used it. It delivers very good results.

http://www.atmel.com/dyn/produ...ily_id=604

Analog-to-Digital Conversion Utilizing the AT89CX051 MCU (6 pages, updated 12/97)
This Application Note describes how to build a low cost A/D converter with the Atmel 20-Pin Microcontrollers.

Hope this helps.

Regards,

Charles Bannister