Hỏi về ngắt timer1

[tdmh.2508]

em có code xuất tần số mà không hiểu đoạn này anh nào pro giúp mình với

//===========================
#int_ext
void dem_xung()
{
so_xung++;
}

#INT_TIMER1
void tao_tre_1s() // Trinh phuc vu ngat tran TIMER1
{
set_timer1(3036); //(65536-3036).1us.8=50 000us
count_t1++; //tran sau 500 000us=0.5us
if(count_t1==2) // 2*500 000=1s
{
tan_so = so_xung;
so_xung = 0;
count_t1 = 0;

}
}
//=====================

chương trình
#include <16f877a.h>
#fuses XT,NOWDT,NOPROTECT,NOLVP
#use delay(clock=4000000)
#use fast_io(d)
#use fast_io(e)
#byte portd=0x08
#bit rs=0x09.0
#bit rw=0x09.1
#bit e=0x09.2
#bit rb0=0x06.0

int16 so_xung,tan_so,count_t1;
int16 tramnghin,chucnghin,nghin,tram,chuc,dv;
int16 u[10]={0x30,0x31,0x32,0x33,0x34,0x35,0x36,0x37,0x38,0x3 9};


#int_ext
void dem_xung()
{
so_xung++;
}

#INT_TIMER1
void tao_tre_1s() // Trinh phuc vu ngat tran TIMER1
{
set_timer1(3036); //(65536-3036).1us.8=50 000us
count_t1++; //tran sau 500 000us=0.5us
if(count_t1==2) // 2*500 000=1s
{
tan_so = so_xung;
so_xung = 0;
count_t1 = 0;

}
}

void ghi_ir()
{
e=1;rs=0;rw=0;e=0;
delay_ms(3);
}
void ghi_dr()
{
e=1;rs=1;rw=0;e=0;
delay_ms(3);
}

void main()
{
{
set_tris_d(0);
set_tris_e(0b000);

enable_interrupts(global);
enable_interrupts(int_ext);
ext_int_edge (H_to_L);
enable_interrupts(INT_TIMER1);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_8);
set_timer1(3036);

portd=0x01;
ghi_ir();
portd=0x0c;
ghi_ir();
portd=0x38;// 2 dong
ghi_ir();

PORTD=0X80;
GHI_IR();
PORTD="T";
GHI_DR();
PORTD="a";
GHI_DR();
PORTD="n";
GHI_DR();
PORTD=" ";
GHI_DR();
PORTD="S";
GHI_DR();
PORTD="o";
GHI_DR();
PORTD=":";
GHI_DR();
}

while(1)
{
tramnghin=tan_so/100000;
chucnghin=(tan_so/10000)%10;
nghin=(tan_so/1000)%10;
tram=(tan_so/100)%10;
chuc=(tan_so/10)%10;
dv=tan_so%10;

portd=0x87;
ghi_ir();
portd=u[tramnghin];
ghi_dr();
portd=u[chucnghin];
ghi_dr();
portd=u[nghin];
ghi_dr();
portd=u[tram];
ghi_dr();
portd=u[chuc];
ghi_dr();
portd=u[dv];
ghi_dr();
portd="H";
ghi_dr();
portd="z";
ghi_dr();

}
}