Based on C51, write a timer to interrupt the system's digital stopwatch display

Here's an example code for a timer interrupt in C51 that can be used to interrupt a digital stopwatch display:

#include <reg52.h>

// Define variables for timer
unsigned char TH0_value;
unsigned char TL0_value;
unsigned char count = 0;

// Define interrupt function for timer
void timer0_ISR (void) interrupt 1 {
    // Reload timer values
    TH0 = TH0_value;
    TL0 = TL0_value;

    // Increment count
    count++;

    // Update stopwatch display (assuming it's a 7-segment display)
    // Code for updating display goes here...
}

void main() {
    // Initialize timer values
    TH0_value = 0x3C; // 60
    TL0_value = 0xB0; // 176

    // Set timer mode and enable interrupt
    TMOD = 0x01; // 16-bit timer mode
    TH0 = TH0_value;
    TL0 = TL0_value;
    ET0 = 1; // Enable timer0 interrupt

    // Enable global interrupts
    EA = 1;

    // Start timer
    TR0 = 1;

    // Loop indefinitely
    while (1) {}
}

This code sets up a timer interrupt using Timer0 in 16-bit mode. The interrupt function `timer0_ISR` is called every time the timer overflows. In this example, the timer is set to overflow every second (60 x 176 = 10560 clock cycles), and the `count` variable is incremented every time the interrupt function is called. The `count` variable can be used to keep track of the number of seconds elapsed, which can then be used to update the stopwatch display. The actual code for updating the display will depend on the specific hardware and software used.