synchronization events (REVT and XEVT) allow synchronization of the CPU or DMA to these conversions,
respectively. Here, the time for this companding depends on the serial bit rate selected.
(1)
(2) (DLB)
From CPU or DMA controller
DXR1
To CPU or DMA controller
DRR1
DX
XSR1
Compress
Expand
DR
RBR1
RSR1
Figure 15-6. Two Methods by Which the McBSP Can Compand Internal Data
15.2.2.3 Reversing Bit Order: Option to Transfer LSB First
Generally, the McBSP transmits or receives all data with the most significant bit (MSB) first. However, certain
8-bit data protocols (that do not use companded data) require the least significant bit (LSB) to be transferred
first. If you set XCOMPAND = 01b in XCR2, the bit ordering of 8-bit words is reversed (LSB first) before being
sent from the serial port. If you set RCOMPAND = 01b in RCR2, the bit ordering of 8-bit words is reversed during
reception. Similar to companding, this feature is enabled only if the appropriate word length bits are set to 0,
indicating that 8-bit words are to be transferred serially. If either phase of the frame does not have an 8-bit word
length, the McBSP assumes the word length is eight bits, and LSB-first ordering is done.
15.2.3 Clocking and Framing Data
This section explains basic concepts and terminology important for understanding how McBSP data transfers
are timed and delimited.
15.2.3.1 Clocking
Data is shifted one bit at a time from the DR pin to the RSR(s) or from the XSR(s) to the DX pin. The time for
each bit transfer is controlled by the rising or falling edge of a clock signal.
The receive clock signal (CLKR) controls bit transfers from the DR pin to the RSR(s). The transmit clock signal
(CLKX) controls bit transfers from the XSR(s) to the DX pin. CLKR or CLKX can be derived from a pin at the
boundary of the McBSP or derived from inside the McBSP. The polarities of CLKR and CLKX are programmable.
, the clock signal controls the timing of each bit transfer on the pin.
D(R/X)
FS(R/X)
CLK(R/X)
B0
B1
B2
B3
B4
B5
B6
B7
A0
A1
ÁÁ
ÁÁ
ÁÁ
Á
Á
Á
ÁÁ
ÁÁ
ÁÁ
Internal
Internal
Figure 15-7. Example - Clock Signal Control of Bit Transfer Timing
Note
The McBSP cannot operate at a frequency faster than ½ the LSPCLK frequency. When driving CLKX
or CLKR at the pin, choose an appropriate input clock frequency. When using the internal sample rate
generator for CLKX and/or CLKR, choose an appropriate input clock frequency and divide down value
(CLKDV) (i.e., be certain that CLKX or CLKR ≤ LSPCLK/2).
15.2.3.2 Serial Words
Bits traveling between a shift register (RSR or XSR) and a data pin (DR or DX) are transferred in a group called
a serial word. You can define how many bits are in a word.
Bits coming in on the DR pin are held in RSR until RSR holds a full serial word. Only then is the word passed to
RBR (and ultimately to the DRR).
During transmission, XSR does not accept new data from DXR until a full serial word has been passed from
XSR to the DX pin.
Multichannel Buffered Serial Port (McBSP)
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
TMS320x2806x Microcontrollers
883
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
Страница 2: ......