SPRUHZ7K August 2015 – April 2024 AM5706 , AM5708 , AM5716 , AM5718 , AM5718-HIREL
Vertical Sync is used to indicate lines that are in the vertical blanking interval. The VSYNC also separates fields or frames. To be spec compliant, VSYNC should be active for a few lines at the bottom of the picture. The exact number of vertical blanking lines at the bottom depends on the specification for the picture format (480i, 480p, 720p, 1080i, 1080p).
Likewise, the number of vertical blanking lines at the top of the picture depends on the video format specification corresponding to the incoming picture.
In the VIP_PARSER, lines associated with an Active VSYNC are stored in the vertical ancillary data buffers using the ANC VPI port to the VPDMA. Lines without an Active VSYNC are stored in the active video Luma and Chroma-pair buffers using the Y and UV VPI ports, respectively, to the VPDMA.
When using HSYNC signaling instead of ACTVID, the VSYNC signal may be derived from an analog source such as an NTSC/PAL decoder. In this case, VSYNC may not transition on the exact cycle as HSYNC. Thus, the VIP_PARSER supports a window region around HSYNC in which VSYNC transitions will be detected. A VSYNC transition occurring within the window is identified the same way as if VSYNC transitioned on the same cycle as HSYNC going active.
The window is defined by a pre-window, which is determined by the VIP_PORT_A[21:16] FID_SKEW_PRECOUNT and VIP_PORT_B[21:16] FID_SKEW_PRECOUNT registers. There is also a post-window that is defined by VIP_PORT_A[29:24] FID_SKEW_POSTCOUNT and VIP_PORT_B[29:24] FID_SKEW_POSTCOUNT for port B. Note that although the configuration registers are named FID_SKEW, they are also used for defining the VSYNC transition window. The window region definition is shown in Figure 9-34.
The results of VSYNC behavior in the transition window are shown in Figure 9-35. A low to high transition in the window is equivalent to VSYNC going low to high on the active HSYNC cycle.
Likewise, a high to low transition in the window is equivalent to VSYNC going high to low on the active HSYNC cycle.
Two transitions of VSYNC within the VSYNC window is not allow and is undefined behavior.
If VSYNC is high throughout the transition window, then the HSYNC line is in vertical blanking.
If VSYNC is low throughout the transition window, then the HSYNC line is in active video.
Note that VSYNC skew generally only applies to input signals that have been sampled from an analog source, as in a NTSC/PAL decoder. If the VSYNC is a VBLANK-type signal or if the sending device is another all-digital IC, then the VSYNC signal does not have a skew since VSYNC will be aligned to HSYNC. In this case, setting FID_SKEW_PRECOUNT = ‘0’ and FID_SKEW_POSTCOUNT = ‘0’ (within PORT_A and PORT_B registers) defines a minimum size window which will capture the value of VSYNC on the same cycle that HSYNC goes active.