DRR
Receive Shift
Register
(RSR)
Transmit Shift
Register
(XSR)
Receive Buffer
(RB)
DXR
DR
DX
To CPU/DMA
From CPU/DMA
Bit reorder
Transmit Buffer
(XB)
Reverse
Reverse
Synchronization and
buffering
Preliminary
Architecture
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11.2 Architecture
11.2.1 McBSP Data Transfer Process
shows the McBSP data transfer paths. McBSP receive operations and transmit operations
are buffered (up to 5 kbytes buffers organized on 32-bit words are used). Input/output registers are
32-bit long words.
Figure 11-2. McBSP Data Transfer Paths
11.2.1.1 Data Transfer Process for 8/12/16/20/24/32-bit Long Words
Receive data arrives on the McBSP.DR pin and is shifted into receive shift register. When a full word is
received, the content of the shift register is copied into receive buffer under condition receive buffer is
not full. When the receive buffer threshold is reached, the McBSP asserts DMA or interrupt request and
the receive buffer content is then transferred to the host (the CPU or the DMA controller reads the data
receive register DRR_REG).
Transmit data is written by the CPU or the DMA controller to data transmit register DXR_REG using the
byte enable input (when a byte is not enabled the byte value in the memory will contain the previous
written value). If there is no previous data in transmit shift register, the value from the transmit buffer is
copied to transmit shift register; otherwise, the content is copied to transmit shift register when the last
bit of the previous data is shifted out on the McBSP.DX pin.
Note that the byte enable is used for transmit data register DXR_REG only and the value is ignored
when the CPU accesses are targeting other registers. When writing to DXR_REG, the bytes are
independently enabled by the byte enable pattern (for example, byte enable is “0001” then only the less
significant byte will be written).
11.2.1.2 Bit Reordering (option to transfer LSB first)
Generally, the McBSP transmits or receives all data with the most significant bit (MSB) first. However,
certain data protocols require the least significant bit (LSB) to be transferred first.
If you set XREVERSE = 01b in XCR2_REG[4:3], the bit ordering of the data words is reversed (LSB
first) before being sent to the serial port.
If you set RREVERSE = 01b in RCR2_REG[4:3], the bit ordering of the data words is reversed during
reception.
This feature is available for all the data formats from 8 up to 32-bit data length.
11.2.1.3 Clocking and Framing Data
This section explains basic concepts and terminology important for understanding how McBSP data
transfers are timed and delimited.
1126
Multichannel Buffered Serial Port (McBSP)
SPRUGX9 – 15 April 2011
© 2011, Texas Instruments Incorporated
Содержание TMS320C6A816 Series
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