Suggest you use exsisting hardware contained within your 80c51 to help with the timing (IE Timers)

8051 Timer Interrupts

This application note provides a programming example of how to use the standard 8051 Timer 0 in an interrupt to generate a timer-tick.

See Keil app note 105

http://www.keil.com/appnotes/docs/apnt_105.asp



There is still something a little weard in response you obtained.. Have you been able to use a debugger or simulator se what is happening?

Joe