65 строки
2.3 KiB
ReStructuredText
65 строки
2.3 KiB
ReStructuredText
==================================
|
|
ASoC Digital Audio Interface (DAI)
|
|
==================================
|
|
|
|
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/p/en_US/business/design
|
|
|
|
|
|
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, while 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 LRCLKs, this allows for simultaneous capture and playback at
|
|
different sample rates.
|
|
|
|
I2S has several different operating modes:-
|
|
|
|
I2S
|
|
MSB is transmitted on the falling edge of the first BCLK after LRC
|
|
transition.
|
|
|
|
Left Justified
|
|
MSB is transmitted on transition of LRC.
|
|
|
|
Right Justified
|
|
MSB is transmitted sample size BCLKs 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 while the Tx and Rx lines are used to transmit and
|
|
receive the audio data. Bit clock usually varies depending on sample rate
|
|
while 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:-
|
|
|
|
Mode A
|
|
MSB is transmitted on falling edge of first BCLK after FRAME/SYNC.
|
|
|
|
Mode B
|
|
MSB is transmitted on rising edge of FRAME/SYNC.
|