51 MCU timer 0 works in mode 2

The 51 MCU timer 0 works in mode 2. The INT0 (P3.2) pin controls the timer/counter to count. The timer/counter 0 overflow interrupt makes the LED connected to the P2.0 pin flash. The timer/counter 0 overflow interrupt is 250μs, and the LED connected to the P2_0 pin turns on and off every 2s. The source code is as follows:
//Original work of 51hei MCU Network, all rights reserved.
#include "reg_c51.h"
#define reload_value 0x06 //The count value is 250, if the clock frequency is 12MHz, it is equivalent to 250μs
unsigned char hex[16]={0xC0,0xF9,0xA4,0xB0,0x99,0x92,0x82,0xF8,0x80,0x90,
0x88,0x83,0xC6,0xA1,0x86,0x8E}; //Hexadecimal decoding table of digital tube
unsigned char n,nn; //Define the interrupt count variable
void main(void) //Main function
{
TMOD &= 0xF0; //Timer/Counter 0 works in mode 2, 8-bit reload mode
TMOD |= 0x0A; //GATE0=1; C/T0#=0; M1=1; M0=0;
TL0 = reload_value; //Set initial value
TH0 = reload_value; //Set initial value
ET0=1; //Enable timer/counter 0 interrupt
EA=1; //Enable total interrupt
TR0=1; //Start timer/counter 0
while(1) //Infinite loop
{
P1=hex[n/10]; //Display the high bit of interrupt count variable n
P0=hex[n%10]; //Display the low bit of interrupt count variable n
}}
void it_timer0(void) interrupt 1 //Timer/counter 0 interrupt function
{
nn=nn++; //Add 1 to nn for each interrupt, and 250μs interval between each interrupt
if (nn==40) //nn=40, which means 40 interrupts, equivalent to 40*250μs =10ms
{
nn=0;
n++; //Add 1 to n every 10ms
if(n==100) //n=100, which is equivalent to 10ms*100=1s
{n=0;
P2_0 = ~P2_0; //Every 1s, invert the level of the P2_0 pin to make the connected LED light flash
}}}