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BT.656 and Y/C Mode Field and Frame Operation
Video Capture Port3-22 SPRU629
3.4.3 Horizontal Synchronization
Horizontal synchronization determines when the horizontal pixel/sample
counter is reset. The EXC and HRST bits in VCxCTL allow you to program the
event that triggers the start of a line. The encoding of these bits is shown in
Table 3–8.
Table 3–8. Horizontal Synchronization Programming
VCxCTL Bit
HMode EXC HRST Horizontal Counter Reset Point
0
0 0 EAV code (H=1) – beginning of horizontal blanking.
1 0 1 SAV code (H=0) – Start of active video.
2 1 0 VCTL1 input active edge – beginning of horizontal blanking or horizontal
sync period. (VCTL1 must be configured as a horizontal control signal.)
3
1 1 VCTL1 input inactive edge – first active pixel on line or end of horizontal
sync. (VCTL1 must be configured as a horizontal control signal.)
HMode 0 is used for BT.656 or Y/C capture (with embedded control) and corre-
sponds to the idea that each line begins with the horizontal blanking period.
It does not align with most standards that start counting with the first active pixel;
therefore, is only useful if capturing of HANC data before the SAV code is
desired. HMode 1 is the default mode and corresponds to most digital video
standards by making the first active pixel pixel0. It has the effect of associating
horizontal blanking periods with the end of the previous line rather than the
beginning of the line, but this is only an issue if you try to capture HANC data.
In either mode, HCOUNT increments on every VCLKIN edge for Y/C operation
and on every other VCLKIN edge for BT.656 operation but only when CAPEN
is active. HCOUNT operation for HMode 0 and HMode 1 is shown in
Figure 3–9.
HMode 2 and HMode 3 are used for BT.656 or Y/C capture without embedded
EAV/SAV code and allow alignment with either the beginning of the horizontal
blanking period or the first active pixel, or the beginning or end of horizontal
sync depending on the VCTL1 input. When VCTL1 is configured as a horizontal
control input, no external CAPEN signal is available so the CAPEN signal is
considered to always be active. HCOUNT operation for HMode 3 and
HMode 4 is shown in Figure 3–10 for VCTL1 operating as either HSYNC or
AVID.