Texas Instruments TMS320C64x DSP Benutzerhandbuch
DMA Operation
Video Port
2-10
SPRU629
A DMA event counter is used to track the number of DMA events generated
in each field as programmed in the VDDISPEVT register. The DISPEVT1 or
DISPEVT2 value (depending on the current display field) is loaded at the start
of each field. The event counter then decrements with each DMA event gener-
ation until it reaches 0, at which point no more DMA events are generated until
the next field begins. Once the last line of data for a field has been requested,
the DMA logic stops generating events until the field is complete in case the
CPU needs to modify the DMA address pointers.
in each field as programmed in the VDDISPEVT register. The DISPEVT1 or
DISPEVT2 value (depending on the current display field) is loaded at the start
of each field. The event counter then decrements with each DMA event gener-
ation until it reaches 0, at which point no more DMA events are generated until
the next field begins. Once the last line of data for a field has been requested,
the DMA logic stops generating events until the field is complete in case the
CPU needs to modify the DMA address pointers.
For BT.656 and Y/C modes, there are three FIFOs, one for each of the Y, Cb,
and Cr color components. Each FIFO generates its own DMA event; therefore,
the DMA event state and FIFO thresholds for each FIFO are tracked indepen-
dently. (The Cb and Cr FIFOs use a threshold value of
and Cr color components. Each FIFO generates its own DMA event; therefore,
the DMA event state and FIFO thresholds for each FIFO are tracked indepen-
dently. (The Cb and Cr FIFOs use a threshold value of
½
VDTHRLD).
2.3.3
DMA Size and Threshold Restrictions
The video port FIFOs are 64-bits wide and always read or write 64 bits at a
time. For this reason, DMA accesses must always be an even number of words
in length. It is expected that in most cases the threshold size is set to the line
length (rounded up to the next doubleword). This always works because differ-
ent lines are not packed together within a doubleword and the Cb and Cr
thresholds (
time. For this reason, DMA accesses must always be an even number of words
in length. It is expected that in most cases the threshold size is set to the line
length (rounded up to the next doubleword). This always works because differ-
ent lines are not packed together within a doubleword and the Cb and Cr
thresholds (
½
VCTHRLDx/VDTHRLD) are always rounded up to the double-
word.
For example, in 8-bit BT.656 capture mode with a line length of 712 (Y), setting
the threshold to the line length results in a VCTHRLD of 712 pixels
the threshold to the line length results in a VCTHRLD of 712 pixels
×
1 bytes/
pixel
×
doubleword/8 bytes = 89 doublewords. The Cb and Cr FIFOs contain
half the data (44.5 doublewords) so their thresholds are set to 45 double-
words. Therefore, the Cb and Cr DMAs each transmit an extra 4 bytes at the
end of each line.
words. Therefore, the Cb and Cr DMAs each transmit an extra 4 bytes at the
end of each line.
If a multihorizontal line length threshold is desired (2 lines, for example) then
the chosen line length must round up to an even number of doublewords so
that it is evenly divisible by 2. If this is not the case, then the Cb and Cr FIFO
transfers are corrupted. For the multiline case, consider the same 8-bit BT.656
capture mode with a line length of 712 (Y). If the threshold is set for 2 lines, this
results in a VCTHRLD value of 2
the chosen line length must round up to an even number of doublewords so
that it is evenly divisible by 2. If this is not the case, then the Cb and Cr FIFO
transfers are corrupted. For the multiline case, consider the same 8-bit BT.656
capture mode with a line length of 712 (Y). If the threshold is set for 2 lines, this
results in a VCTHRLD value of 2
×
89 = 178 doublewords. The actual Cb/Cr
line length is 44.5 doublewords that requires a length of 45. To transfer 2 lines
requires 2
requires 2
×
45 = 90 doublewords. However, for this VCTHRLD, the DMA
logic would calculate the Cb/Cr threshold size as 178/2 = 89 doublewords,
which is 1 doubleword off. This can be corrected by increasing the line length
to 720 pixels (and ignoring the extra captured pixels) or decreasing it to
704 pixels.
which is 1 doubleword off. This can be corrected by increasing the line length
to 720 pixels (and ignoring the extra captured pixels) or decreasing it to
704 pixels.