
÷
Frame
Pulse
1
0
1
0
1
0
÷
Frame pulse
detection and
clock
synchronization
CLKXP bit
mcbspi_clkx
mcbspi_clkr
CLKRP bit
CLKSM bit
McBSPi_ICLK
CLKS
CLKSP bit
GSYNC bit
FSR_int
SCLKME bit
CLKG
FSG
FWID bit
FPER bit
CLKGDV bit
CLKSRG
mcbsp-034
Public Version
McBSP Functional Description
www.ti.com
21.4.3 McBSP SRG
The McBSP module contains an internal SRG that can be used to generate an internal data clock (CLKG)
and an internal frame-synchronization signal (FSG). CLKG can be used for bit shifting on the data receive
pin (mcbspi_dr) and/or the data transmit pin (mcbspi_dx). FSG can be used to initiate frame transfers on
mcbspi_dr pin and/or mcbspi_dx pin.
is a conceptual block diagram of the SRG.
Figure 21-39. Conceptual Block Diagram of the Sample Rate Generator
The source clock for the SRG (labeled CLKSRG in the diagram) can be supplied by either the interface
clock (McBSPi_ICLK), or the functional clock (CLKS input), or by an external pin (mcbspi_clkx, or
mcbspi_clkr). The source is selected with the McBSPi.
[7] SCLKME bit and the
McBSPi.
[13] CLKSM bit.
If a pin or CLKS signal is used, the polarity of the incoming signal can be inverted with the appropriate
polarity bit (McBSPi.
[14] CLKSP bit, McBSPi.
[1] CLKXP
bit, or McBSPi.
[0] CLKRP bit).
The SRG has a three-stage clock divider that gives CLKG and FSG programmability.
The three stages provide:
•
Clock divide-down: The source clock (CLKSRG) is divided according to the
McBSPi.
[7:0] CLKGDV field to produce CLKG signal
•
Frame period divide-down: CLKG is divided according to the McBSPi.
FPER field to control the period from the start of a frame-pulse to the start of the next pulse
•
Frame-synchronization pulse-width countdown: CLKG cycles are counted according to the
McBSPi.
[15:8] FWID field to control the width of each frame-synchronization
pulse
NOTE:
The McBSP module cannot operate at an internal functional frequency faster than L4
interface frequency divided by 2. Choose an input clock frequency and a
McBSPi.
[7:0] CLKGDV value such that CLKG is less than or equal
to L4 interface frequency divided by 2.
In addition to the three-stage clock divider, the sample rate generator has a frame-synchronization pulse
detection and clock synchronization module that allows synchronization of the clock divide down with an
incoming frame-synchronization pulse on the mcbspi_fsr pin. This feature is enabled or disabled with the
McBSPi.
[15] GSYNC bit.
3104
Multi-Channel Buffered Serial Port
SWPU177N – December 2009 – Revised November 2010
Copyright © 2009–2010, Texas Instruments Incorporated