Cirrus Logic AN282 - Manual (Page 3)

AN282
AN282REV1 3
2. THE SERIAL AUDIO INTERFACE FORMATS
There are several de-facto industry standards or formats that define the required alignments and signal polarities
between the Left/Right Clock, Serial Clock and the Serial audio data.
2.1 Left-Justified Data Format
The Left-Justified (LJ) format probably got its name from the relationship of the audio data and the Left/Right
Clock. Refer to
Figure 3 and notice that the MSB of the audio word is coincident with the leading transitions
in the Left/Right Clock, or left-justified within the frame. One of the advantages of this format is that it is word-
length independent and works properly with audio data words of 16 to 32 bits, assuming there is a sufficient
number of serial clocks per Left/Right clock cycle.
Another important aspect of the LJ format is that the audio data for the left channel is presented on the serial
data line during the high portion of the Left/Right Clock, and the right channel is presented during the low
portion.
Figure 3 Standard Left-Justified Format
2.2 Right-Justified Data Format
Referring to Figure 4 , it is relatively apparent that the Right-Justified (RJ) format is very similar to the Left-
Justified format, with the exception of the placement of the audio word within the frame. Notice that the trail-
ing edge of the LSB is coincident with the trailing edge of the Left/Right Clock, or right-justified within the
audio frame. The primary disadvantage of this format is that the interface is not word-length independent.
As a result, any device that is receiving data in a Right-Justified format must be aware of the transmitted
audio word length. This explains why most Digital-to-Analog (D/A) converters are required to support mul-
tiple right-justified formats.
As with the Left-Justified format, the audio data for the left channel is presented on the serial data line during
the high portion of the Left/Right Clock, and the right channel is presented during the low portion.
Figure 4 Standard Right-Justified Format
L R C K
S C L K
L e f t C h a n n e l R i g h t C h a n n e l
SDATA +3 +2 +1 +5 +4 -1 -2 -3 -4 -5 +3 +2 +1 +5 +4 -1 -2 -3 -4 MSB LSB MSB LSB
LRCK
SCLK
L e f t C h a n n e l
SDATA 6 +5 +4 +3 +2 +1 LSB 7 MSB -1 -2 -3 -4 -5
3 2 c l o c k
0
R i g h t C h a n n e l
MSB -1 -2 -3 -4 -5 6 +5 +4 +3 +2 +1 LSB 7