eFUSE is a non-volatile one-time-programmable technology used for selected configuration settings. The fuse link is programmed (or burned or blown) by flowing a large current for a specific amount of time. User-programmable eFUSEs can be programmed with the Xilinx configuration tools.
IMPORTANT: eFUSE bits are one-time programmable (OTP). After they are programmed, they cannot be un-programmed.
For example, if access to a register is disabled, it cannot be re-enabled. The FPGA logic can access only the FUSE_USER register value. All other eFUSE bits are inaccessible from the FPGA logic. Table: eFUSE Storage Location Advantages and Disadvantages identifies eFUSE storage location advantages and disadvantages.