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XEMPTY
XRDY
DX
FSX
CLKX
DXR-to-XSR copy (B)
Write of DXR (C)
C5
C6
C7
B0
B1
B2
B3
B4
B5
B6
A1
B7
A0
DXR-to-XSR copy (C)
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B4
B5
XEMPTY
B6
B7
B0
B1
B2
B3
B4
B5
B6
A1
XRDY
DX
FSX
B7
A0
CLKX
Write of DXR (C)
DXR-to-XSR copy (B)
Architecture
1219
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multichannel Buffered Serial Port (McBSP)
25.2.7.5.4 Transmit Empty: XEMPTY
XEMPTY indicates whether the transmitter has experienced underflow. Either of the following conditions
causes XEMPTY to become active (XEMPTY = 0):
•
During transmission, DXR has not been loaded since the last DXR-to-XSR copy, and all bits of the
data element in XSR have been shifted out on DX.
•
The transmitter is reset (XRST = 0 or the device is reset), and then restarted.
During an underflow condition, the transmitter continues to transmit the old data in DXR for every new
frame sync signal FSX (generated by an external device, or by the internal sample rate generator) until a
new element is loaded into DXR by the CPU or the EDMA controller. XEMPTY is deactivated (XEMPTY =
1) when this new element in DXR is transferred to XSR. In the case when the FSX is generated by a
DXR-to-XSR copy (FSXM = 1 in PCR and FSGM = 0 in SRGR), the McBSP does not generate any new
frame sync until new data is written to the DXR and a DXR-to-XSR copy occurs.
When the transmitter is taken out of reset (XRST = 1), it is in a transmit ready (XRDY = 1) and transmit
empty (XEMPTY = 0) condition. If DXR is loaded by the CPU or the EDMA controller before FSX goes
active, a valid DXR-to-XSR transfer occurs. This allows for the first element of the first frame to be valid
even before the transmit frame sync pulse is generated or detected. Alternatively, if a transmit frame sync
is detected before DXR is loaded, 0s are output on DX.
shows a transmit underflow condition. After B is transmitted, B is retransmitted on DX if you
fail to reload the DXR before the subsequent frame synchronization.
shows the case of
writing to DXR just before a transmit underflow condition that would otherwise occur. After B is
transmitted, C is written to DXR before the next transmit frame sync pulse occurs.
Figure 25-28. Transmit Empty
Figure 25-29. Transmit Empty Avoided