I think you'll have to have a underlying program/script calling it every 15 minutes. Like
Code:
#!/bin/sh
while [ 1 -eq 1 ] ; do
/home/user/readvalue >>readvalue.txt
sleep 15m
done
Save these lines into a file, and chmod u+x <file>. Then call it from your /etc/rc.d/rc.local and it will be started at every boot.
<EDIT>
Ha! This was post no. 300 for me - cheers!
</EDIT>