An269 – Cirrus Logic AN269 User Manual
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AN269REV1
AN269
Note that the number of available video clocks can also be derived by adding up the number of clocks in
each region, but this approach will guarantee a more accurate line frequency.
The value of NumVideoClocks will be the total number of “available” VIDCLK periods for each region of
time in the horizontal line. In order to visualize this quantity, see
. Note that the NumVideoClocks
quantity represents the total number of VIDCLKs per horizontal line, and therefore will be HClksTotal+1.
Now, the number of VIDCLK periods required for the active region (i.e., region with valid pixel data) can
be determined. In the following equation, ActiveVidClks represents the total number of VIDCLKs that will
occur while outputting pixel data (VIDCLKs per Pixel is usually 1):
ActiveVidClks = (VIDCLKs per Pixel) * (horizontal resolution)
We will now discuss each of the regions in more detail. Each region is labeled by the appropriate length
of time, in VIDCLKs. A diagram of this is shown in
. The time from the HSYNC signal becoming
active to the time it becomes inactive is LoadHighVidClks. The time from the HSYNC signal becoming
inactive until the VCSYNC signal becomes active (on the first frame) is noted as FrameHoldVidClks. The
time from VCSYNC becoming active until the first valid SPCLK is LoadCPVidClks. The time from the last
SPCLK until the next HSYNC is CPLoadVidClks.
HSYNC
SPCLK
t
HSYNCH
Single Horizontal Line
DATA
One SPCLK per Horizontal Pixel
VCSYNC
(Horizontal
Line 1 ONLY)
t
SPCLK
t
HSYNCL
t
HSYNCSPCLK
t
SPCLKHSYNC
t
HVCSYNC
t
VCHSYNC
Count = HClkTotal
Count = 0
Horizontal Line
Counter Value
HSyncStart
HSyncStop
HSyncStop
HActiveStart
HActiveStop
Count = HClkTotal - 1
Count = HClkTotal - 2
Count = HClkTotal
Count = 0
Figure 15. Horizontal Line for Frame Type 1 Displays