Data Transmit Registers (DXR1 and DXR2)
12-3
McBSP Registers
SPRU592E
12.2 Data Transmit Registers (DXR1 and DXR2)
For transmission, the CPU or the DMA controller writes data to one or both of
the data transmit registers (see Figure 12
2). If the serial word length is 16 bits
or smaller, only DXR1 is used. If the word length is larger than 16 bits, both
DXR1 and DXR2 are used, and DXR2 holds the most significant bits. Each
frame of transmit data in the McBSP can have one phase or two phases, each
with its own serial word length.
DXR1 and DXR2 are I/O mapped registers; they are accessible at addresses
in I/O space.
Figure 12
−
2. Data Transmit Registers (DXR1 and DXR2)
DXR2
15
0
High part of transmit data (for 20-, 24- or 32-bit data)
R/W-0
DXR1
15
0
Transmit data (for 8-, 12-, or 16-bit data) or Low part of receive data (for 20-, 24- or 32-bit data)
R/W-0
Legend:
R = Read; W = Write; -
n
= Value after reset
12.2.1 How Data Travels From the DXRs to the Data Transmit (DX) Pin
If the serial word length is 16 bits or fewer, data written to DXR1 is copied to
transmit shift register 1 (XSR1). From XSR1, the data is shifted onto the DX pin
one bit at a time.
If the serial word length is more than 16 bits, data written to DXR1 and DXR2
is copied to both transmit shift registers (XSR2, XSR1). From the XSRs, the
data is shifted onto the DX pin one bit at a time.
If companding is used during the transfer from DXR1 to XSR1
(XCOMPAND = 10b or 11b), the McBSP compresses the 16-bit data in DXR1
to 8-bit data in the
µ
-law or A-law format in XSR1. If companding is disabled,
the McBSP passes data from the DXR(s) to the XSR(s) without modification.
The XSRs are not accessible. They are not mapped to I/O space like the
DXRs.
Содержание TMS320VC5509
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