When I try to perform the simulation on SPI_ACCESS using ISim, I see the following error:
** Failure:DRC Error : In SPI_ACCESS opcode is not recognized
User(VHDL) Code Called Simulation Stop
In process unisim_VITAL.vhd:186631
What does "opcode" refer to? Why does this error pop up?
The opcode means the first byte of the command that you send to the MOSI of SPI_ACCESS.
The error messages indicate that the opcode is either unsupported in simulation or an invalid opcode.
The supported opcode list can be found in Table 1-4 of the "Spartan-3AN FPGA In-System Flash User Guide" (UG333):