Other things to check.....

Make sure you use RETI instruction to exit the interrupt routine.

Make sure that Basic52 is not expecting to use Timer 0 or Timer 1 for its own uses.

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Note that a 50 uSec interrupt rate on a timer in an 8052 is a quite fast interrupt depending upon the clock speed you have. look carefully at all the instruction timings in this interrupt routine to make sure that the total interrupt pprocessing time it a bit shorter than 50 usec. If your interrupt was taking 48 usec you would be leaving very little remaining time for the processor to run Basic 52.
Michael Karas