Transmitting and receiving

The I²S module supports both transmission (TX) and reception (RX) of serial data. In both cases the serial data is shifted synchronously to the clock signals SCK and LRCK.

TX data is written to the SDOUT pin on the falling edge of SCK, and RX data is read from the SDIN pin on the rising edge of SCK. The most significant bit (MSB) is always transmitted first.

Note: When starting a transmission in master mode, the first frame is filled with zeros.

TX and RX are available in both Master and Slave modes and can be enabled/disabled independently in the CONFIG.TXEN and CONFIG.RXEN.

Transmission and/or reception is started by triggering the START task. With transmission enabled in CONFIG.TXEN), the TXPTRUPD event will be generated for every number of transmitted data words given by RXTXD.MAXCNT. Each data word contains one or more samples. The TXPTRUPD event is generated just before MAXCNT number of data words have been transmitted. Similarly, with reception enabled in CONFIG.RXEN, the RXPTRUPD event will be generated for every received data word given by RXTXD.MAXCNT. The RXPTRUPD event is generated just after MAXCNT number of data words have been received.

The FRAMESTART event is generated synchronously to the active LRCK edge at the beginning of a frame after transmitting RXTXD.MAXCNT data words. The initial FRAMESTART event is generated at the first active edge of LRCK after the START task has been triggered. The FRAMESTART event is only defined for transmitting full left and right sample pairs. If MAXCNT is configured so that the frame ends between the left and right sample pairs, the FRAMESTART event is not generated. This occurs for the following combinations of SWIDTH and MAXCNT:

Table 1. Restrictions on combinations of SWIDTH and MAXCNT for correct FRAMESTART
SWIDTH	MAXCNT restriction
24Bit	Only even numbers (2,4,6, etc)
32	Only even numbers (2, 4, 6, etc)
24BitIn32	Only even numbers (2, 4, 6, etc)

Transmitting and receiving. CONFIG.FORMAT = Aligned, CONFIG.SWIDTH = 8Bit, CONFIG.CHANNELS = Stereo, RXTXD.MAXCNT = 1. — Figure 2. Transmitting and receiving. CONFIG.FORMAT = Aligned, CONFIG.SWIDTH = 8Bit, CONFIG.CHANNELS = Stereo, RXTXD.MAXCNT = 1