Reading from software serial with an interrupt
You can't do it directly with an interrupt. SoftwareSerial already uses an interrupt to do the actual reception - the received byte being placed in a circular buffer.

The only thing you could do would be to modify the source code for SoftwareSerial so that instead of storing the byte in the buffer it calls your custom code instead.



Ok, so you're NOT using SoftwareSerial, you're using (something which I have never heard of) "NeoSWSerial". That has the ability to attach an interrupt (by adding it to the reception interrupt ISR), and the README.md file with that library gives you an example of how to use it:

#include &lt;NeoSWSerial.h&gt;
NeoSWSerial ss( 4, 3 );

volatile uint32_t newlines = 0UL;

static void handleRxChar( uint8_t c )
{
  if (c == '\n')
    newlines++;
}

void setup()
{
  ss.attachInterrupt( handleRxChar );
  ss.begin( 9600 );
}


So just read the character in there, set a flag to say you have a new "mode" arrived, and in your loop react to that flag and run the right function for that mode.

If you want to terminate a mode early you will have to program your modes to also react to the "new mode" flag and terminate. The interrupt cannot do that for you.