AN2682 - Application note

General description

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General description

1.1

I2S protocol

I2S (IC to IC Sound) is an audio data transfer standard using a three-line bus for serial and

synchronous data transmission.

Data are transmitted on the SD line (Serial Data) in Little Endian format (MSB first). Data

length is not limited (usually 16/20/24/32/64 bits). Data are synchronized by the SCK (Serial

Clock) rising or falling edge for the transmitter, and falling edge only for the receiver. Refer to

Figure 1.

Data represent stereo digital sound, so each sample contains two words, the right channel

sample and the left channel sample. Instead of using two data channels, muxing is

performed by transmitting each word in half a sampling period and that allows, by doubling

the sampling rate, to transmit two words per period.

A control signal WS (Word Select) is then used to determine if the word being sent is the

right or the left one. This signal also determines the beginning and the end of the data: there

is no need to fix the data length. Receiver and transmitter data lengths can therefore be

different, as well as the right and left data lengths.

WS is synchronized either on the rising or the falling edge of SCK and precedes the MSB by

one SCK period in order to have enough time for storing and shifting operations.

As in the SPI protocol, there must be a master and a slave. The master controls and

provides the SCK clock and the WS signal, while the slave only sends or receives data. The

master can be the receiver or the transmitter or a third element (Controller). Refer to

Figure 2.

Figure 1.

I2S Philips protocol waveforms 16/32-bit

MSB

LSB MSB

CK

WS

SD

Left channel

Right channel

16-bit or 32-bit

Transmission

Reception