Ok, this would be for PicBasic Pro. Tested on an 877, but it should still work with a 628.
How you do it in Proton, I haven't got a clue.
Code:
@ DEVICE WDT_OFF   ; Disable Watch-Dog timer

DEFINE OSC 4

LED     VAR PORTB.1
StopPin VAR PORTB.0
INTEDG  VAR OPTION_REG.6   ; 0 = falling edge, 1 = rising
INTE    VAR INTCON.4       ; External interrupt Enable bit
INTF    VAR INTCON.1       ; External interrupt Flag
Delay   VAR WORD

INPUT  StopPin             ; Is already default, but just to make sure
INTEDG = 0                 ; Interrupt on falling edge
INTE = 1                   ; Enable External Interrupts

Main:
    Toggle LED
    For Delay = 1 to 500
        PAUSE 1
        IF StopPin = 1 THEN
           LOW  LED            ; Turn OFF anything that will draw current
           INTF = 0            ; Clear the Interrupt Flag 
           @ SLEEP             ; enter Low Power mode
           GOTO MAIN
        endif
    NEXT Delay
Goto Main
HTH,