There is no A2B specification, per se, but the protocol is described to an extent in the Technical Reference Manual and in the data sheet specifications.
There are numerous development boards available to provide the ability to mock up all functional use case combinations (master, local-powered slave for mics or I2S/TDM or both, and bus-powered slave for mics or I2S/TDM or both). These are the EVAL-AD2428WD1BZ, EVAL-AD2428WB1BZ, EVAL-AD2428WG1BZ, and EVAL-AD2428WC1BZ A2B evaluation boards. Also available are the SHARC Audio Module (SAM, the ADZS-SC589-MINI) and its associated class-D amplifier board (ADZS-AUDIOA2BAMP).
