hi,
from the mailing-list archive:
have a look at [phasorshot~] (externals / tof - include in pd-extended with [import tof]). it does exactly what you want.
from the help file:
phasorshot~ is exactly like a phasor except that you can disabling it's looping, therefore making it a one shot lookup signal.
Of course line~ does something similar, but with phasorshot~ you can change the speed DURING the ramp.
how-to install pdmtl abstractions: http://www.vimeo.com/psc/videos
pat