From Famitracker Wiki
3xx is the effect that enables automatic portamento. It can be used on any channel except DPCM. xx selects the time it takes to sweep from one note to the next; larger values mean shorter sweep time (i.e. faster portamento). 00 disables portamento. It should be noted that any volume envelope does not reset during portamento. For best results, use instruments with constant volume.
Using this effect on the Konami VRC7 chip will eliminate any retriggering of notes.
C-4 00 F --- D-4 -- - 302 --- -- - ---
C-4 is note, 00 is instrument number, F is volume. Sound will start from C-4 and then pitch will slowly change to D-4.
Portamento example — portamento with same instrument and with additional instrument