Hi,
What kind of a channel is it for which u r setting up ATP? If u set it as daily then u will be able to select 15 minutes say for example: you selected 1500 to 1515..You need to create one more ATP for that channel from 1530 to 1545 and so on..but this will be cumbersome..Instead if you have a File, SFTP channel then set the polling interval for 15 minutes..It will work.
Regards,
Sriram