0
1
1
0
CLKXM
0
1
FSX pin
FSR pin
CLKR pin
CLKX pin
FSG
FSX_int
CLKX_int
Frame selection
Clock selection
CLKG
FSR_int
CLKS pin
Internal clock source (see Figure 5)
DXR to XSR
FSGM
0
1
FSR_int
CLKR_int
FSRP
1
0
0
1
FSRM
FSRM and GSYNC
FSRP
0
1
FSXP
FSXP
FSXM
FSXM
generator
Sample
rate
Receive
Transmit
DLB
CLKRM
CLKRM
CLKXM
CLKRP
CLKRP
CLKXP
CLKXP
DLB
Architecture
1196
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multichannel Buffered Serial Port (McBSP)
25.2.5 Clock, Frames, and Data
The McBSP has several ways of selecting clocking and framing for both the receiver and transmitter.
Clocking and framing can be sent to both portions by the sample rate generator. Each portion can select
external clocking and/or framing independently.
is a block diagram of the clock and frame
selection circuitry.
Figure 25-2. Clock and Frame Generation
25.2.5.1 Frame and Clock Operation
Receive and transmit frame sync pulses (FSR/X), and clocks (CLKR/X), can either be generated internally
by the sample rate generator (see
) or be driven by an external source. The source of
frame sync and clock is selected by programming the mode bits, FS(R/X)M and CLK(R/X)M respectively,
in the pin control register (PCR). FSR is also affected by the GSYNC bit in the sample rate generator
register (SRGR), see
for details.
When FSR and FSX are inputs (FSXM = FSRM = 0), the McBSP detects them on the internal falling edge
of clock, CLKR_int and CLKX_int, respectively (see
). The receive data arriving at the DR pin
is also sampled on the falling edge of CLKR_int. These internal clock signals are either derived from an
external source via the CLK(R/X) pins or driven by the sample rate generator clock (CLKG) internal to the
McBSP.
When FSR and FSX are outputs driven by the sample rate generator, they are generated (transition to
their active state) on the rising edge of the internal clock, CLK(R/X)_int. Similarly, data on DX is output on
the rising edge of CLKX_int.
The FSRP, FSXP, CLKRP, and CLKXP bits in PCR configure the polarities of FSR, FSX, CLKR, and
CLKX. All frame sync signals (FSR_int and FSX_int) internal to the serial port are active high. If the serial
port is configured for external frame synchronization (FSR/FSX are inputs to the McBSP) and FSRP =
FSXP = 1, the external active (low) frame sync signals are inverted before being sent to the receiver
signal (FSR_int) and transmitter signal (FSX_int). Similarly, if internal synchronization is selected
(FSR/FSX are outputs and GSYNC = 0), the internal active (high) sync signals are inverted if the polarity
bit FS(R/X)P = 1, before being sent to the FS(R/X) pin.
shows this inversion using XOR gates.