Misunderstandings of using the interrupt on change of pic16f877a

       Working principle: RB4:RB7 is the input state of the level change interrupt pin and compares it with the value of the latch of PORTB read last time. When the two sets of data are different, an interrupt occurs. When the level of part of these four pins changes, the final OR will be taken. As long as the input state of one pin changes, an interrupt change will occur.

       Note: When entering the interrupt service subroutine, you must read or write the value of the PORTB latch, so that the comparison of the two sets of data will end, and then clear the interrupt flag. If the value of PORTB is not read, the comparison of the two sets of data will always exist. It is useless to clear the interrupt flag (RBIF) first, and RBIF will always be set. For example, the following example:

/*This is a photoelectric encoder (code disk) reading using INT interrupt and level change interrupt*/

void 
interrupt on_change()
{
 if(INTF) 
 {
 f_r++;
 INTF=0;
 }
 if(RBIF&&RB4) //High level count
 { 
 RB4=~RB4; //Read or write the value of PORTB latch first
 f_l++;
 RBIF=0; //Clear interrupt flag (RBIF)
 }
 else {RB4=~RB4; RBIF=0;} //Low level
 
}

The following program is the problem that has been bothering us during the experiment, that is, there is no interrupt.

Incorrect usage, the latch value is not read

void 
interrupt on_change()
{
 if(INTF) 
 {
 f_r++;
 INTF=0;
 }
 if(RBIF&&RB4) 
 { 
 f_l++;
 RBIF=0;//
 }
 else {RBIF=0;}
 
}