Category: SoCs
tool:-
Device: Cyclone® V

(reference material)

Cyclone V Device Datasheet
https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/cyclone-v/cv_51002.pdf
(Item of Quad SPI Flash Timing Characteristics,
Table 43. Quad Serial Peripheral Interface (SPI) Flash Timing Requirements for Cyclone V Devices)

Of the two clocks (l4_mp_clk/qspi_ref_clk) supplied to the QSPI Controller, this is qspi_ref_clk.

(Reference material)

https://www.intel.com/content/dam/altera-www/global/en_US/pdfs/literature/hb/cyclone-v/cv_5v4.pdf
(Functional Description of the Quad SPI Flash Controller > Clocks item,
Or search for qspi_ref_clk)

The qspi_ref_clk clock is the reference clock for the quad SPI controller and
is used to serialize the data and drive the external SPI interface.

Quoted from <<Functional Description of the Quad SPI Flash Controller > Clocks section>>