The SSTL_2 pad set is a full complement of I/O, power, and spacer cells (total of 14 cells) that are necessary to assemble a padring by abutment. Since the SSTL_2 normally operates with its own isolated power domain (2.5V), a “rail-splitter” support cell (SPP_RS_005_25V) is included to allow the designer to easily break the lines that should not connect to the rest of the padring, while allowing VDD and VSS to be continuous within the padring.