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Our ASIC comes up reading the flash in legacy single-bit mode. Is the legacy SPI command accepted if the flash QPI mode is enabled? If the state of the configuration registers is not known, do we need to send a command multiple times in single, dual, quad, and QPI modes respectively?
Legacy SPI commands will not be accepted if the flash QPI mode is enabled. Only the commands sent in 4-4-4 format are accepted. To use QPI mode, the host must be able to send the command in 4-4-4 format.
Most of the commands are supported in QPI mode. The commands not supported in QPI mode are invalid when QPI mode is enabled. Refer to the datasheet for the list of commands that are not supported in QPI mode. For the commands supported in QPI mode, the host must send commands in 4-4-4 format when QPI mode is enabled.
Many system designs have host and flash in the same mode configuration, that is, both host and flash are configured as default QPI enabled or disabled. There is no problem for the host to access flash after power-up.
If the host does not know the default flash mode, a suggested algorithm is to try to read the SFDP signature (that is, SFDP Header 1st DWORD – “SFDP”) in 4-4-4 format. If that fails, try 2-2-2 format, and if that also fails, try 1-1-1 format. In this manner, the host can identify the flash mode, and then decide the correct command format for the flash accesses afterward.