Cirrus Logic AN269 - Manual (Page 57)

AN269
AN269REV1 57
Once the VIDCLKDIV register has been setup, the actual VIDCLK rate can be used for setting up the horizontal
LOAD/HSYNC pulse timing. To do this, we first determine the time spent on a single line. This is done by first de-
termining the line period and then the number:
LinePeriod = 1 / [(refresh rate) * (vertical resolution)] = 1 / (73 Hz * 240) = 57077 ns
NumVideoClocks = LinePeriod/VidClkPeriod = 57077 ns / 172 ns = 331
ActiveVidClks = (VIDCLKs/Pixel) * (horizontal resolution) = 1 VIDCLK/Pixel * 320 Pixels = 320 VIDCLKs
LoadHighVidClks = (tHSYNCH / VidClkPeriod) + 1 = (50 ns / 172 ns) + 1 = 1 VIDCLK
FrameHoldVidClks = (tHVCSYNC / VidClkPeriod) + 1 = (30 ns / 172 ns) + 1 = 1 VIDCLK
LoadCPVidClks = [(tHSYNCSPCLK - tHVCSYNC) / VidClkPeriod] + 1 = [(70 ns - 30 ns) / 172 ns] + 1 = 1 VIDCLK
CPLoadVidClks = (tSPCLKHSYNC / VidClkPeriod) + 1= (0 ns / 172 ns) + 1 = 1 VIDCLK
AvailableVidClks = NumVideoClocks - ActiveVidClks - LoadHighVidClks - FrameHoldVidClks -
LoadCPVidClks - CPLoadVidClks
AvailableVidClks = 331 - 320 - 1 - 1 - 1 - 1 = 7 VIDCLKs
Distribute:
LoadHighVidClks = 3
FrameHoldVidClks = 3
LoadCPVidClks = 3
CPLoadVidClks = 2
HClksTotal = NumVideoClocks - 1 = 331 - 1 = 330
HSyncStart = LoadHighVidClks + FrameHoldVidClks - 1 = 3 + 3 - 1= 5
HSyncStop = FrameHoldVidClks - 1 = 3 - 1 = 2
HActiveStart = HClksTotal - LoadCPVidClks - 1= 330 - 3= 327
HActiveStop = HClksTotal - LoadCPVidClks - ActiveVidClks - 1= 330 - 3 - 320 = 7
HClksStart = HClksTotal - LoadCPVidClks - 6 = 330 - 3 - 6 = 321
HClkStop = HClksTotal - LoadCPVidClks - ActiveVidClks - 6 = 330 - 3 - 320 - 6 = 1
Note that the blank output is not used, so 0 can be assigned to the horizontal blank timing registers:
HBlankStart = 0
HBlankStop = 0
VLinesTotal = (vertical resolution) - 1 = 240 - 1 = 239
VSyncStart = VLinesTotal = 239
VSyncStop = VLinesTotal - 1 = 239 - 1 = 238
VActiveStart = VLinesTotal = 239
VActiveStop = VLinesTotal + 1 = 239 + 1 = 240
VClkStart = VLinesTotal = 239
VClkStop = VLinesTotal + 1 = 239 + 1 = 240
VBlankStart = 0
VBlankStop = 0