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ALTERA_AVALON_UART_INIT( SERIALE_INT, seriale_int );
ALTERA_AVALON_UART_INIT( SERIALE_EXT, seriale_ext );
ALTERA_AVALON_UART_INIT( SERIALE_FUT, seriale_fut );[/b]
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Your problem is exactly as I diagnosed the lines above are initialising the driver for each of your UARTs so when the interrupt goes off for one of your UARTs the driver interrupt handler get's called and your ISR does not get a look in. There is no easy method to turn off the drivers at present you will have to remove the _INIT and _INSTANCE macros from alt_sys_init.c. The problem is that this is an auto generated file which will be re-generated under your feet.
You would be much better off using the provided driver and if you don't understand it step through it. Otherwise you will have to start from the alt_main entry point example which hands you complete control of the machine, with no drivers etc. This does have drawbacks though which are coverred in the manual.