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 15-3. Companding Processes
15.2.2.1 Companding Formats
For reception, the 8-bit compressed data in RBR1 is expanded to left-justified 16-bit data in DRR1. The receive
sign-extension and justification mode specified in RJUST is ignored when companding is used.
For transmission using μ-law compression, the 14 data bits must be left-justified in DXR1 and that the remaining
two low-order bits are filled with 0s as shown in
µ
-law format in DXR1
00
Value
1-0
15-2
Figure 15-4. μ-Law Transmit Data Companding Format
For transmission using A-law compression, the 13 data bits must be left-justified in DXR1, with the remaining
three low-order bits filled with 0s as shown in
000
2-0
Value
15-3
A-law format in DXR1
Figure 15-5. A-Law Transmit Data Companding Format
15.2.2.2 Capability to Compand Internal Data
If the McBSP is otherwise unused (the serial port transmit and receive sections are reset), the companding
hardware can compand internal data. This can be used to:
• Convert linear to the appropriate μ-law or A-law format
• Convert μ-law or A-law to the linear format
• Observe the quantization effects in companding by transmitting linear data and compressing and re-
expanding this data. This is useful only if both XCOMPAND and RCOMPAND enable the same companding
format.
shows two methods by which the McBSP can compand internal data. Data paths for these two
methods are used to indicate:
• When both the transmit and receive sections of the serial port are reset, DRR1 and DXR1 are connected
internally through the companding logic. Values from DXR1 are compressed, as selected by XCOMPAND,
and then expanded, as selected by RCOMPAND. RRDY and XRDY bits are not set. However, data is
available in DRR1 within four CPU clocks after being written to DXR1.
The advantage of this method is its speed. The disadvantage is that there is no synchronization available to
the CPU and DMA to control the flow. DRR1 and DXR1 are internally connected if the (X/R)COMPAND bits
are set to 10b or 11b (compand using μ-law or A-law).
• The McBSP is enabled in digital loopback mode with companding appropriately enabled by RCOMPAND
and XCOMPAND. Receive and transmit interrupts (RINT when RINTM = 0 and XINT when XINTM = 0) or
Multichannel Buffered Serial Port (McBSP)
882
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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