15.8.11 Set the Transmit Companding Mode
Table 15-58. Register Bits Used to Set the Transmit Companding Mode
Register
Bit
Name
Function
Type
Reset
Value
XCR2
4-3
XCOMPAND
Transmit companding mode
R/W
00
Modes other than 00b are enabled only when the appropriate XWDLEN
is 000b, indicating 8-bit data.
XCOMPAND = 00b
No companding, any size data, MSB
transmitted first
XCOMPAND = 01b
No companding, 8-bit data, LSB
transmitted first (for details, see
XCOMPAND = 10b
μ-law companding, 8-bit data, MSB
transmitted first
XCOMPAND = 11b
A-law companding, 8-bit data, MSB
transmitted first
15.8.11.1 Companding
Companding (COMpressing and exPANDing) hardware allows compression and expansion of data in either
μ-law or A-law format. The companding standard employed in the United States and Japan is μ-law. The
European companding standard is referred to as A-law. The specifications for μ-law and A-law log PCM are part
of the CCITT G.711 recommendation.
A-law and μ-law allow 13 bits and 14 bits of dynamic range, respectively. Any values outside this range are set
to the most positive or most negative value. Thus, for companding to work best, the data transferred to and from
the McBSP via the CPU or DMA controller must be at least 16 bits wide.
The μ-law and A-law formats both encode data into 8-bit code words. Companded data is always 8 bits wide;
the appropriate word length bits (RWDLEN1, RWDLEN2, XWDLEN1, XWDLEN2) must therefore be set to 0,
indicating an 8-bit wide serial data stream. If companding is enabled and either of the frame phases does not
have an 8-bit word length, companding continues as if the word length is 8 bits.
illustrates the companding processes. When companding is chosen for the transmitter,
compression occurs during the process of copying data from DXR1 to XSR1. The transmit data is encoded
according to the specified companding law (A-law or μ-law). When companding is chosen for the receiver,
expansion occurs during the process of copying data from RBR1 to DRR1. The receive data is decoded to
twos-complement format.
From CPU or DMA controller
DXR1
To CPU or DMA controller
DRR1
16
16
DX
8
8
XSR1
Compress
Expand
DR
RBR1
RSR1
Figure 15-53. Companding Processes for Reception and for Transmission
15.8.11.2 Format for Data To Be Compressed
For transmission using μ-law compression, make sure the 14 data bits are left-justified in DXR1, with the
remaining two low-order bits filled with 0s as shown in
.
µ
-law format in DXR1
00
Value
1-0
15-2
Figure 15-54. μ-Law Transmit Data Companding Format
Multichannel Buffered Serial Port (McBSP)
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
TMS320x2806x Microcontrollers
945
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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