It’s an example of 8051 Timer0 as Second Counter on 7 Segment.
In this example we have configured 8051 timer0 for time calculation. There are 4 different modes for timer0 in 8051. We have selected 8 bit auto reload mode. In this mode timer0 hardware is automatically reset upon overflow to a predefined value and again starts counting up. Timer0 counts up 1 after 12 clocks of main system clock oscillator. So if we are using 12 MHz crystal then timer0 input frequency will be 1 MHz.
Here in this project system clock is of 24 MHz and timer0 input clock is 2 MHz. It is reset to value 6 after each overflow. Simply each timer interrupt is occurred after 125 micro seconds. And so in this way if 8 interrupts has occurred then 1 millisecond has passed and if 8000 interrupts has occurred then 1 second has passed and so on. In this way we can calculate hours and minutes also. This time in seconds is updated on 2 digit 7 segment displays regularly. For detail of 7 segment interfacing please see 8051 To 7 Segment Display Interfacing.
For basic operation & components used in an 8051 based microcontroller based systems see
8051 Basic LED Flasher.