单片机C52程序，通过按键控制led灯的亮度，我自己写了一个想按一下就把占空比调大到50%，但最开始灯不亮

#include<reg52.h>#define uchar unsigned char#define uint unsigned intsbit PWM = P1^0; sbit S1=P3^6; uint count=0, preset=10; void timer_init(){ TMOD = 0x00; TCON=0x00; TH0 = 0xFF; TL0 = 0xEF; EA = 1; ET0 = 1; TR0 = 1; }void delay(void){ unsigned int i; for(i=5000;i>0;i--) ;}void timer0_irpt()interrupt 1{ TH0 = 0xFF; TL0 = 0xEF; count++; if(count+1 <= preset) { PWM = 0; } else { PWM = 1; } if(count == 100) { count = 0; }}void main(void){ while(1) { if(S1==0) { delay(); if(S1==0) { if(F0==0) { F0=1; preset=50; } } } else { F0=0; } }}