As specified in the I2S protocol, data bits are left-adjusted in the Word Select slot, with the MSB transmitted first, starting one clock period after 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, a low level for the left channel and a high level for the right channel.
In I2S format, typical configurations are described below. These configurations do not list all necessary settings, but only basic ones. Other configuration settings are to be done as per requirement such as clock and DMA configurations.
Case 1: I2S 16-bit compact stereo
- Slot size configured as 16 bits (CLKCTRL0.SLOTSIZE = 0x1)
- Number of slots configured as 2 (CLKCTRL0.NBSLOTS = 0x1)
- Data size configured as 16-bit compact stereo (SERCTRL0.DATASIZE = 0x05)
- Data delay from Frame Sync configured as 1-bit delay (CLKCTRLn.BITDELAY = 0x01)
- Frame Sync Width configured as HALF frame (CLKCTRLn.FSWIDTH = 0x01)
Case 2: I2S 24-bit stereo Transmitterwith 24-bit slot
- Slot size configured as 24 bits (CLKCTRL0.SLOTSIZE = 0x2)
- Number of slots configured as 2 (CLKCTRL0.NBSLOTS = 0x1)
- Data size configured as 24 bits (SERCTRL0.DATASIZE = 0x01)
- Data delay from Frame Sync configured as 1-bit delay (CLKCTRLn.BITDELAY = 0x01)
- Frame Sync Width configured as HALF frame (CLKCTRLn.FSWIDTH = 0x01)
In both cases, it will ensure that Word select signal is 'low level' for the left channel and 'high level' for the right channel.
The length of transmitted words can be chosen among 8, 16, 18, 20, 24, and 32 bits by writing the Data Word Size bit group in the Serializer Control mregister (SERCTRLm.DATASIZE).
If the slot allows for more data bits than the number of bits specified in the respective DATASIZE field, additional bits are appended to the transmitted or received data word as specified in the SERCTRLm.EXTEND field. If the slot allows less data bits than programmed, the extra bits are not transmitted, or received data word is extended based on the EXTEND field value.