how to control output voltage of 89c51 to fed led light

LEDs are currnt devices, you want to control current, NOT voltage

Nothing, of course, to do with Keil: electronicsclub.info/leds.htm

A possible way is LED Dimming by Pulse Width Modulation (PWM).

Can anyone plz provide me .asm or .c program code generating pwm of diff dutycycle such as 60%,70%,80%.

I got the award for this function. I let you use it.

void WMP_OUTPUT( int PCT , int CYCULS)
{
  int COUNT;

LOOP1:
  if ( CYCULS == 0 )
    goto END;

  CYCULS = CYCULS - 1;

  COUNT = 0;

  P1.1 = 1;

LOOP2:
  if ( CNT == PCT )
    goto BRANCH1;

  CNT = CNT + 1;

  goto LOOP2;

BRANCH1:

  P1.1 = 0;

LOOP3:
  if ( CNT == 100 )
    goto BRANCH2;

  CNT = CNT + 1;

  goto LOOP3;

BRANCH2:

  goto LOOP1;
}

Was that the Pretend You're Programming In Assembler Award?

Or the Most Unnecessary Uses Of 'goto' Award?

Or what?!

I got it 32 years ago. I was 7.

PCA cookbook application-notes.digchip.com/.../27-46415.pdf