Например можна так сделать:
#define __PIC24HJ64GP206__
#include <p24Hxxxx.h>
#include <timer.h>
void __attribute__(( __interrupt__)) _T1Interrupt( void);
int main (void)
{
OpenTimer1 (T1_ON & T1_GATE_OFF & T1_PS_1_8 & T1_SYNC_EXT_OFF & T1_SOURCE_INT, 0x3F);
ConfigIntTimer1(T1_INT_PRIOR_3 & T1_INT_ON);
while (1);
}
//void __attribute__(( __interrupt__)) _T1Interrupt( void)
void _T1Interrupt( void )
{
/* reset Timer 1 interrupt flag */
IFS0bits.T1IF = 0;
}