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DX
DR
Compand
RSR[1,2]
Compress
Expand
XSR[1,2]
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DXR[1,2]
DRR[1,2]
RBR[1,2]
To CPU/DMA controller
From CPU/DMA controller
Figure 15-2. McBSP Data Transfer Paths
15.2.1.1 Data Transfer Process for Word Length of 8, 12, or 16 Bits
If the word length is 16 bits or smaller, only one 16-bit register is needed at each stage of the data transfer paths.
The registers DRR2, RBR2, RSR2, DXR2, and XSR2 are not used (written, read, or shifted).
Receive data arrives on the DR pin and is shifted into receive shift register 1 (RSR1). Once a full word is
received, the content of RSR1 is copied to receive buffer register 1 (RBR1) if RBR1 is not full with previous data.
RBR1 is then copied to data receive register 1 (DRR1), unless the previous content of DRR1 has not been read
by the CPU or the DMA controller. If the companding feature of the McBSP is implemented, the required word
length is 8 bits and receive data is expanded into the appropriate format before being passed from RBR1 to
DRR1. For more details about reception, see
.
Transmit data is written by the CPU or the DMA controller to data transmit register 1 (DXR1). If there is no
previous data in transmit shift register (XSR1), the value in DXR1 is copied to XSR1; otherwise, DXR1 is
copied to XSR1 when the last bit of the previous data is shifted out on the DX pin. If selected, the companding
module compresses 16-bit data into the appropriate 8-bit format before passing it to XSR1. After transmit
frame synchronization, the transmitter begins shifting bits from XSR1 to the DX pin. For more details about
transmission, see
.
15.2.1.2 Data Transfer Process for Word Length of 20, 24, or 32 Bits
If the word length is larger than 16 bits, two 16-bit registers are needed at each stage of the data transfer paths.
The registers DRR2, RBR2, RSR2, DXR2, and XSR2 are needed to hold the most significant bits.
Receive data arrives on the DR pin and is shifted first into RSR2 and then into RSR1. Once the full word is
received, the contents of RSR2 and RSR1 are copied to RBR2 and RBR1, respectively, if RBR1 is not full. Then
the contents of RBR2 and RBR1 are copied to DRR2 and DRR1, respectively, unless the previous content of
DRR1 has not been read by the CPU or the DMA controller. The CPU or the DMA controller must read data from
DRR2 first and then from DRR1. When DRR1 is read, the next RBR-to-DRR copy occurs. For more details about
reception, see
For transmission, the CPU or the DMA controller must write data to DXR2 first and then to DXR1. When new
data arrives in DXR1, if there is no previous data in XSR1, the contents of DXR2 and DXR1 are copied to XSR2
and XSR1, respectively; otherwise, the contents of the DXRs are copied to the XSRs when the last bit of the
previous data is shifted out on the DX pin. After transmit frame synchronization, the transmitter begins shifting
bits from the XSRs to the DX pin. For more details about transmission, see
15.2.2 Companding (Compressing and Expanding) Data
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.
Multichannel Buffered Serial Port (McBSP)
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
TMS320x2806x Microcontrollers
881
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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