| ASoC currently supports the three main Digital Audio Interfaces (DAI) found on |
| SoC controllers and portable audio CODECS today, namely AC97, I2S and PCM. |
| |
| |
| AC97 |
| ==== |
| |
| AC97 is a five wire interface commonly found on many PC sound cards. It is |
| now also popular in many portable devices. This DAI has a reset line and time |
| multiplexes its data on its SDATA_OUT (playback) and SDATA_IN (capture) lines. |
| The bit clock (BCLK) is always driven by the CODEC (usually 12.288MHz) and the |
| frame (FRAME) (usually 48kHz) is always driven by the controller. Each AC97 |
| frame is 21uS long and is divided into 13 time slots. |
| |
| The AC97 specification can be found at :- |
| http://www.intel.com/design/chipsets/audio/ac97_r23.pdf |
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| |
| I2S |
| === |
| |
| I2S is a common 4 wire DAI used in HiFi, STB and portable devices. The Tx and |
| Rx lines are used for audio transmission, whilst the bit clock (BCLK) and |
| left/right clock (LRC) synchronise the link. I2S is flexible in that either the |
| controller or CODEC can drive (master) the BCLK and LRC clock lines. Bit clock |
| usually varies depending on the sample rate and the master system clock |
| (SYSCLK). LRCLK is the same as the sample rate. A few devices support separate |
| ADC and DAC LRCLK's, this allows for simultaneous capture and playback at |
| different sample rates. |
| |
| I2S has several different operating modes:- |
| |
| o I2S - MSB is transmitted on the falling edge of the first BCLK after LRC |
| transition. |
| |
| o Left Justified - MSB is transmitted on transition of LRC. |
| |
| o Right Justified - MSB is transmitted sample size BCLK's before LRC |
| transition. |
| |
| PCM |
| === |
| |
| PCM is another 4 wire interface, very similar to I2S, which can support a more |
| flexible protocol. It has bit clock (BCLK) and sync (SYNC) lines that are used |
| to synchronise the link whilst the Tx and Rx lines are used to transmit and |
| receive the audio data. Bit clock usually varies depending on sample rate |
| whilst sync runs at the sample rate. PCM also supports Time Division |
| Multiplexing (TDM) in that several devices can use the bus simultaneously (this |
| is sometimes referred to as network mode). |
| |
| Common PCM operating modes:- |
| |
| o Mode A - MSB is transmitted on falling edge of first BCLK after FRAME/SYNC. |
| |
| o Mode B - MSB is transmitted on rising edge of FRAME/SYNC. |
