SPI NOR flash densities over 128 Mb require more than the
traditional 24-bit addressing that was standard before the introduction of 256 Mb
and larger flashes. Flash vendors use various methods to support 32-bit addressing
that may enable the 24-bit read commands to operate as a 32-bit read command. For
example, a nonvolatile bit might be set in the flash that causes the flash to expect
four address bytes after a 03h command. These
methods should not be enabled for the flash devices used to configure Spartan
UltraScale+ FPGAs.
The solution supported by the Spartan UltraScale+ FPGAs require the flash to configure from a 24-bit address when the SPI_24 configuration mode is used or to configure from a 32-bit address when the SPI_32 configuration mode selection is used.