Electronic – AVR ATmega: can I trigger external interrupt by setting corresponding INTF flag bit

As @angelatlarge said your first task is to view the disassembly of your interrupt vector to determine how many cycles your code actually takes. Second, you have to realize that there is additional overhead associated with a call to an interrupt vector. Here is an extract from the ATMega328P datasheet, section 7.7.1 - this should be representative of all AVRs:

The interrupt execution response for all the enabled AVR interrupts is four clock cycles minimum. After four clock cycles the program vector address for the actual interrupt handling routine is executed. During this four clock cycle period, the Program Counter is pushed onto the Stack. The vector is normally a jump to the interrupt routine, and this jump takes three clock cycles. If aninterrupt occurs during execution of a multi-cycle instruction,this instruction is completed before the interrupt is served. If an interrupt occurs when the MCU is in sleep mode, the interrupt exe-cution response time is increased by four clock cycles. This increase comes in addition to the start-up time from the selected sleep mode. A return from an interrupt handling routine takes four clock cycles. During these four clock cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is incremented by two, and the I-bit in SREG is set.

So, to sum it up:

1. 4 clock cycles to push PC onto the stack
2. 3 clock cycles to jump to interrupt handler
3. Up to 3 clock cycles if a multi-cycle instruction is interrupted (AVR instruction set table in the same document indicates that 4 cycles is the longest instruction)
4. 4 more clock cycles when exiting the interrupt to restore PC, increment SP, etc.

This adds anywhere from 11-14 additional cycles of overhead on top of however many cycles your interrupt code takes.

This is probably because there are other functions on pin B1 that need to be disabled before you can use it as a digital input.

In the datasheet we find table 11-4, shown below:

A quick look at that register in the datasheet shows that it is 1 by default, which means analogue input mode:

You will need to clear this bit before you can use it as a digital input.

You will also need to reset INTCON3bits.INT1IF to 0 in your interrupt service routine or the handler will keep firing and you will never escape it.

If you are trying to measure PWM you might like to read section 20.2 "Capture Mode" in the datasheet. This uses the Enhanced Capture and Compare Peripheral. It acts like your edge selection interrupt but will also store the value of TMR1 or TMR3, allowing you to make measurements of the period or duty cycle (or both).