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CLK
FS
AXR[n]
(A)
Word n−1
right channel
Word n
left channel
Word n+1
right channel
MSB
LSB
MSB
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CLK
(B)
Slot 0
Slot 0
Slot 0
Slot 1
Slot 1
Slot 1
(2 bit delay)
Frame sync:
(1 bit delay)
Frame sync:
Frame sync:
(0 bit delay)
Frame
sync
(A)
(B)
(B)
1085
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multichannel Audio Serial Port (McASP)
Figure 24-7. TDM Format Bit Delays from Frame Sync
A
FS duration of slot is shown. FS duration of single bit is also supported.
B
Last bit of last slot of previous frame. No gap between this bit and the first bit of slot 0 is allowed.
In a typical audio system, one frame of data is transferred during each data converter sample period f
s
. To
support multiple channels, the choices are to either include more time slots per frame (thus operating with
a higher bit clock rate), or to use additional data pins to transfer the same number of channels (thus
operating with a slower bit clock rate).
For example, a particular six channel DAC may be designed to transfer over a single serial data pin
AXR[n] as shown in
. In this case the serial clock must run fast enough to transfer a total of
6 channels within each frame period. Alternatively, a similar six channel DAC may be designed to use
three serial data pins AXR[0,1,2], transferring two channels of data on each pin during each sample period
(
). In the latter case, if the sample period remains the same, the serial clock can run three
times slower than the former case. The McASP is flexible enough to support either type of DAC.
24.0.18.1.2 Inter-IC Sound (I2S) Format
The Inter-IC Sound (I2S) format is used extensively in audio interfaces. The TDM transfer mode of the
McASP supports the I2S format when configured to 2 slots per frame.
I2S format is specifically designed to transfer a stereo channel (left and right) over a single data pin
AXR[n]. "Slots" are also commonly referred to as "channels". The frame width duration in the I2S format is
the same as the slot size. The frame signal is also referred to as "word select" in the I2S format.
shows the I2S protocol.
The McASP supports transfer of multiple stereo channels over multiple AXR[n] pins.
Figure 24-8. Inter-IC Sound (I2S) Format
A
1 to 16 data pins may be supported.