Hello

The 8051 controller does not reset the internal interrupt level state, if the program is restarted with a LJMP 0.

So, if a LJMP 0 is done from inside a interrupt function, of a nested interrupt function, the problem you describe will show.

You can (just as an example) add to your startup code following code:

LabelA:
    SJMP  LabelC
LabelB:
    RETI
LabelC:    
    MOV   R0, #4      ; The maximum nested interrupt level possible
    LCALL LabelB      ; Execute a RETI 
    DJNZ  R0, LabelC

 

This hint does not mean, that a real reset wouldn't be better; but this was not your question.

Best regards
Juergen