19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
FSG
CLKG
Frame width: (FWID + 1)
CLKG
Frame period: (FPER + 1)
CLKG
Architecture
1204
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multichannel Buffered Serial Port (McBSP)
When the sample rate generator comes out of reset, FSG is in an inactive (low) state. After this, when
FRST = 1 in SPCR and FSGM = 1 in SRGR, frame sync signals are generated. The frame width value
(FWID + 1) is counted down on every CLKG cycle until it reaches 0 when FSG goes low. Thus, the value
of FWID + 1 determines an active frame pulse width ranging from 1 to 256 data bit clocks. At the same
time, the frame period value (FPER + 1) is also counting down, and when this value reaches 0, FSG goes
high again, indicating a new frame is beginning. Thus, the value of FPER + 1 determines a frame length
from 1 to 4096 data bits. When GSYNC = 1 in SRGR, the value of FPER does not matter.
shows a frame of 16 CLKG periods (FPER = 15 or 0000 1111b).
It is recommended that FWID be programmed to a value less than (R/X)WDLEN1/2.
Figure 25-9. Programmable Frame Period and Width
25.2.5.4.2 Receive Frame Synchronization Selection: DLB and FSRM
shows how you can select various sources to provide the receive frame synchronization signal.
Note that in digital loopback mode (DLB = 1 in the serial port control register (SPCR)), the transmit frame
sync signal is used as the receive frame sync signal and that DR is internally connected to DX.
NOTE:
FSR_int and FSX_int are shown in
Table 25-5. Receive Frame Synchronization Selection
DLB Bit
in SPCR
FSRM Bit
in PCR
GSYNC Bit
in SRGR
Source of Receive Frame
Synchronization
FSR Pin Function
0
0
X
External frame sync signal drives the FSR
input pin, whose signal is then inverted as
determined by FSRP before being used as
FSR_int.
Input.
0
1
0
Sample rate generator frame sync signal
(FSG) drives FSR_int, FRST = 1.
Output. FSG is inverted as determined by
FSRP before being driven out on the FSR
pin.
0
1
1
Sample rate generator frame sync signal
(FSG) drives FSR_int, FRST = 1.
Input. The external frame sync input on FSR
is used to synchronize CLKG and generate
FSG.
1
0
0
FSX_int drives FSR_int. FSX is selected as
shown in
High impedance.
1
X
1
FSX_int drives FSR_int and is selected as
shown in
Input. External FSR is not used for frame
synchronization but is used to synchronize
CLKG and generate FSG since GSYNC = 1.
1
1
0
FSX_int drives FSR_int and is selected as
shown in
Output. Receive (same as transmit) frame
synchronization is inverted as determined
by FSRP before being driven out.