Underflow in the Transmitter
4-9
McBSP Exception/Error Conditions
SPRU592E
4.5 Underflow in the Transmitter
The McBSP indicates a transmitter empty (or underflow) condition by clearing
the XEMPTY bit in SPCR2. Either of the following events activates XEMPTY
(XEMPTY = 0):
-
DXR1 has not been loaded since the last DXR-to-XSR copy, and all bits
of the data word in the XSR(s) have been shifted out on the DX pin.
-
The transmitter is reset (by forcing XRST = 0 in SPCR2, or by a DSP reset)
and is then restarted.
In the underflow condition, the transmitter continues to transmit the old data
that is in the DXR(s) for every new transmit frame-sync signal until a new value
is loaded into DXR1 by the CPU or the DMA controller.
Note:
If both DXRs are needed (word length larger than 16 bits), the CPU or the
DMA controller must load DXR2 first and then load DXR1. As soon as DXR1
is loaded, the contents of both DXRs are copied to the transmit shift registers
(XSRs). If DXR2 is not loaded first, the previous content of DXR2 is passed
to the XSR2.
XEMPTY is deactivated (XEMPTY = 1) when a new word in DXR1 is
transferred to XSR1. If FSXM = 1 in PCR and FSGM = 0 in SRGR2, the
transmitter generates a single internal FSX pulse in response to a
DXR-to-XSR copy. Otherwise, the transmitter waits for the next frame-sync
pulse before sending out the next frame on DX.
When the transmitter is taken out of reset (XRST = 1), it is in a transmitter
ready (XRDY = 1 in SPCR2) and transmitter empty (XEMPTY = 0) state. If
DXR1 is loaded by the CPU or the DMA controller before internal FSX goes
active high, a valid DXR-to-XSR transfer occurs. This allows for the first word
of the first frame to be valid even before the transmit frame-sync pulse is
generated or detected. Alternatively, if a transmit frame-sync pulse is detected
before DXR1 is loaded, zeros will be output on DX.
Содержание TMS320VC5509
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