I'm a student having trouble with hardware interupts in C. I wrote a program that uses the two external interupts on the 89c420 which seem to work perfectly. When I attempt to use the serial uart interupt I get some odd behavior. I notice the two pins I have going high while the interupts are active intermitenly go high for seemingly random periods of time then back to low. The second issue I have noticed is when I set a pin high in the interupt handler for the serial line I get a pin that toggles high to low very rapidly even though there is no traffic on the serial line. My first guess was the code was placed incorectly into the interupt vector table in the 89c420 so I made a new interupt handler that simply called my original. This did not solve the problem so I figured I would post here to see if anyone had any sugestions that I might be able to follow up on.

If anyone would like to view the code or list file generated let me know and I will post it. The compiler I'm using the ProView compiler from Franklin.