"after internal timer time up"

I presume you mean one of the 8051's hardware timers?

In your Interrupt Service Routine (ISR) for the hardware timer interrupt, just set a flag; eg,

time_up = 1;

Then the rest of your program can check this flag, and take appropriate action when it's set; eg,

if( time_up )
{
   time_up = 0;
   // do the necessary
   :
}