The Mode pins M[2:0] should be tied
and static during configuration. The FPGA reads the modes pins at power-up to
determine which configuration mode to use. JTAG mode is most commonly used for
debugging. In Spartan
UltraScale+ FPGAs you
must set the mode to JTAG if you want to configure in this configuration mode. If a
non-JTAG mode is selected at power-up, the system must wait for a configuration
attempt (either successful or failure) before the user can send attempt a JTAG
configuration or the user must issue a JTAG instruction JPROG.