2.4.3 Setting the Quad Enable (QE) Bit

For almost all memory manufacturers, the QE bit is non-volatile and must be set before performing any SPI command that requires the 4 I/O lines. This is the only persistent setting that the ROM code may change in the internal registers of the QSPI NOR Flash memory. All other settings are kept unchanged.

The procedure to set this QE bit is manufacturer-specific and may also change between different memory models of the same manufacturer.

Again, the ROM code first checks the SFDP tables to find out the right procedure. If no SFDP table is found, then the ROM code looks up in its own hard-coded table to get the procedure to be executed.

More precisely, the ROM code reads bits[22:20] in DWORD15 from the Basic Flash Parameter Table (refer to JEDEC JESD216B specification) to select and then execute the relevant procedure, if any, to set the QE bit.