How measure(Voltage reference points) SPI flash Data Output parameters (tCLQV,tCLQX)

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Joined Nov 28, 2012
we are using MT25QU256ABA8ESF SPI flash as boot flash. It operates in QPI mode @ 120Mhz.

As part of DVT we are measuring timing parameters for both Data Input & Data Output at Flash pins (No option to measure at the controller pins).
Data Input parameters tDVCH(setup), tCHDX(hold) are clear. they have defined thresholds as 0.3VCC, 0.7VCC based on rising /falling.
But for Data output parameters like
tCLQV ( Clock Into Output Data Valid)
tCLQX (Output hold time)
There is no clarity on the Voltage levels.
For tCLQV where i can make sure Output data became valid, Is it for HIGH can i take VCC and for LOW can i take GND .