29.6.2.1.4 SCKn Clock Frequency

When the Serial Clock (SCKn) is generated from GCLK_I2S_n and both CLKCTRLn.MCKSEL and CLKCTRLn.SCKSEL are zero, the Serial Clock (SCKn) frequency is GCLK_I2S_n frequency divided by (MCKDIV+1).

i.e.