https://forums.xilinx.com/t5/7-Series-FPGAs/about-the-7-series-FPGA-flip-flop-types/td-p/370527
FDCE Primitive: D Flip-Flop with Clock Enable and Asynchronous Clear
FDPE Primitive: D Flip-Flop with Clock Enable and Asynchronous Preset
FDRE Primitive: D Flip-Flop with Clock Enable and Synchronous Reset
FDSE Primitive: D Flip-Flop with Clock Enable and Synchronous Set
