Digital voltmeter design

A design topic,
Digital voltmeter design.
Second, design task
Digital voltmeter is used for measuring voltage meter, the subject that requires the application SCM, A/D converter and other parts digital voltage measurement instrument.
Three, the design requirements
http://www.siliconray.com/atmega128-mcu-standard-kit.html
1, basic requirements
(1) the MCS-51 series microcontroller as the core device, form a digital voltmeter.
(2) measuring voltage range is divided into two levels: DC 0 ~ 5 V and DC 0 ~ 51.2 V, minimum measuring voltage error 50 mV.
(3) using three tube voltage LED digital display, two integers, 1 decimal places.
(4) write detailed design report.
(5) give all circuits and the source program.
2, play a part
(1) add two function keys used to set up and down voltage limit.
(2) when measuring voltage over ? up and down