Hi Pavel,
Define "one digital interface".
For a simplex transfer, i.e. from mcu to DAC, or from ADC to mcu (and in I2S we are always talking about stereo, so it's technically a dual-DAC or dual-ADC, but nobody cares with this level of detail), you need a pair of clocks (bit clock (BCK, CLK, the marking varies, the Philips's official I2S spec surely has some, I am lazy to look it up, and other manufacturers merrily ignore the spec as usually) and a framing clock (left-right clock LRCK, word-select clock WSCK etc.), plus one data line (sometimes marked as DI/DO for data in/out, or SI/SO serial in/out). Some ADC/DAC require an additional master clock, some can generate it internally, some can generate all clocks, leaving the mcu in the role of slave. Some ADC/DAC chips also have a "side-channel", e.g. I2C, through which you configure the innards of ADC/DAC e.g. gain or filters; simpler ones are fixed or have only a few settings selected simply by logic levels on dedicated pins.
CODECs, i.e. chips which integrate ADC with DAC, feature duplex transfer, and have basically two data lines with only one shared pair of clocks.
The SPI/I2S module in STM32 features a simplex I2S (maybe except 'H7, the SPI there is a beast and I am not interested in it). Some STM32 like the classic 'F4xx (e.g. 'F407) have on paper duplex I2S, but that's in fact accomplished by an extra stripped-down module marked SPI_Ex, i.e. it has removed SPI and master functions, so it's I2S slave only, which has the clocks internally connected to its respective full-fledged SPI/I2S module.
The SAI module consists from a couple of simplex units, but its two halves can be synchronized internally, making it effectively duplex.
JW
