In SCK ratio = 2 scenarios, a successful read operation occurs when the SPI data, launched on the falling-edge of SCK, is captured on the second rising-edge of ext_spi_clk. This means that the data should be available and stable at the time the capture happens on the second rising edge.
The worst case scenario occurs when:
• SCK has max delay on the line (that is, Max delay of STARTUP)
• SPI drives the data with max Tco
Fmax4 = fn {STARTUP delay max, Tco-max, board routing}
Thus the actual Fmax is the one that is most restrictive amongst Fmax2, Fmax3 and Fmax4.