You have to display data in two different modes and use Timer interrupts. If you want I will write a program for you. Which MCU you want to use? PIC, AVR, 8051 ? What crystal frequency will you use ?
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You have to use a button to toggle the display mode. If button status is 0 then adc value is deisplayed and if button is pressed then button status will be 1 and it will display set value.
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What kind of display (Common Cathode or Common Anode) you want to use ? Do you want to display integer data (ADC value) or floating point value ? Can you use MAX7219 type 8 digit 7 segment display which uses SPI for communication ?