Dear Douglas,

From 8051 family programmer's guide:

INTERRUPTS:
To use any of the interrupts in the 80C51 Family, the following three steps must be taken.
1. Set the EA (enable all) bit in the IE register to 1.
2. Set the corresponding individual interrupt enable bit in the IE register to 1.
3. Begin the interrupt service routine at the corresponding Vector Address of that interrupt. See Table below.

In addition, for external interrupts, pins INT0 and INT1 (P3.2 and P3.3) must be set to 1, and depending on whether the
interrupt is to be level or transition activated, bits IT0 or IT1 in the TCON register may need to be set to 1.
ITx = 0 level activated
ITx = 1 transition activated

Regards,
Silviu