Adam Ford wrote:
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I can put a volt meter on the INT0 pin and I can see when it goes low and when it's high.
...

Seems, you are wrong:
The interrupt sense this pin also. It's working as input, not as output. And you must trigger it from another source, e.g. with a key.

The two external interrupt inputs on the 8051 are provided to react on events from outside.


Peter