The outputs of the TIEHI and TIELO cells are driven through diffusion to provide isolation from the power and ground rails for better ESD protection. The standard cell abstractmethodology assumes that the TIEHI and TIELO cells are used to tie off any
inputs to power and ground. If these cells are not used and the router is allowed
to drop vias on the power rail, DRC errors or shorts may result.
Decap is composed of a PMOS and NMOS device to form decoupling capacitor between VDD and VSS rails so as to reduce the voltage bounce on the power rails.