2.5.1.4.10 QSPI NOR Flash Boot

Hardware Considerations

The ROM code configures the hardware so that:

• the QSPI controller uses SPI Mode 0 (CPOL = 0 and CPHA = 0),
• the QSPIx_SCK clock frequency is ≤ 50 MHz,
• QSPIx_SCK and QSPIx_CS do not use any internal pull-up/pull-down resistor,
• each QSPIx_IO{0,1,2,3} uses the PIO controller’s internal pull-up resistor.

Software Considerations

Before reading any data, the ROM code sends a software reset to the QSPI NOR memory. Then the ROM code looks for the Serial Flash Discoverable Parameters (SFDP) of the QSPI NOR memory, if available, to learn the parameters (instruction op code, timing settings) required to read the user-programmed boot file.

If SFDP tables are not available, the ROM code uses hard-coded values as fallback settings to read the boot file.

The ROM code supports any QSPI NOR memory which can provide its Serial Flash Discoverable Parameters (SFDP) as defined in the JEDEC^® JESD216B standard.

The supported revisions of this JEDEC standard are:

• JESD216 (version 1.0)
• JESD216 rev. A (version 1.5)
• JESD216 rev. B (version 1.6)

Refer to the QSPI NOR memory data sheet to check compliance with any of the above JEDEC JESD216 standard revisions/versions.

QSPI NOR Memories with SFDP (JEDEC JESD216x Compliant)

The ROM code reads the memory SFDP tables to learn the factory settings (instruction op code, number of dummy cycles, etc.). The ROM code also reads bits[22:20] in DWORD15 from the Basic Flash Parameter table (refer to the JEDEC JESD216B specification) to select and then execute the relevant procedure, if any, to set the Quad Enable (QE) bit in some internal register of the QSPI NOR memory.

For most memory manufacturers, this QE bit is non-volatile and must be set before performing any Quad SPI command. This is the only persistent setting that the ROM code may change in the internal registers of the QSPI NOR memory. All other settings are kept unchanged.

Refer to the QSPI NOR memory data sheet to find which value was chosen by the memory manufacturer and written into the SFDP tables.

Finally, the ROM code reads the boot file from the data area of the QSPI NOR memory, and then continues its boot procedure.

QSPI NOR Memories without SFDP

This section only applies when the ROM code fails to read the SFDP tables from the QSPI NOR memory.

The ROM code reads the JEDEC ID of the QSPI NOR memory, and then selects the read settings based on the manufacturer ID (first byte of the JEDEC ID) from the following hard-coded values:

Those hard-coded parameters give a last chance to the ROM code to boot from a QSPI NOR memory in either Normal mode or XIP (Continuous Read) mode.