47.6.5 Left-Justified Reception and Transmission Sequence
As specified in the I2S protocol, data bits are left-justified in the word select time slot, with the MSB transmitted first, starting at the same clock period as the transition on the word select line.
Data bits are sent on the falling edge of the serial clock and sampled on the rising edge of the serial clock. The word select line indicates the channel in transmission, with a low level for the right channel and a high level for the left channel.