请问视频输入的2个时钟选项哪个是最好的？

嗨鲍勃
感谢您的快速回复。
我正在使用Spartan 6部分，所以我将查看OSERDES2原语。
我还需要运行一些逻辑，在没有检测到视频输入时创建一个闪屏。
当检测到像素视频时钟时，启动屏幕禁用并且视频被序列化并输出。
运行此闪屏并使用外部时钟驱动LVDS逻辑同时连续采样视频像素时钟以进行活动是否合理，然后在检测到像素时钟切换输出时切换到视频？
史蒂夫



以下为原文

Hi Bob
 
Thanks for the speedy reply.  
 
I am using a Spartan 6 part so i'll look into the OSERDES2 primitive.  
 
I also need to run some logic which creates a splash-screen when no video input is detected.  When the pixel video clock is detected the splash screen disables and the video is serialized and output.
 
Would it be sensible to run this splashscreen and drive the LVDS logic with an external clock while continuously sampling the video pixel clock for activity, then switch to the video when the pixel clock is detected switch the outputs?
 
 
Steve

运行此闪屏并使用外部时钟驱动LVDS逻辑同时连续采样视频像素时钟以进行活动是否合理，然后在检测到像素时钟切换输出时切换到视频？
如果输入视频像素时钟不是恒定时钟源，那么您显然必须提供备用时钟。
有几种设计方法可以适用于您描述的应用程序 - 只需选择一个并进行前进。
就个人而言，我不会切换时钟源。
我将为内部逻辑提供一个固定的固定时基，并设计一条路径，用于将输入视频信号桥接到内部时基。
典型实现使用过采样（和输入像素时钟边沿检测）或FIFO。
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以下为原文

Would it be sensible to run this splashscreen and drive the LVDS logic with an external clock while continuously sampling the video pixel clock for activity, then switch to the video when the pixel clock is detected switch the outputs?
 
If the input video pixel clock is not a constant clock source, then you clearly must provide an alternative clock.  There are several design approaches which can work for the application you describe -- just pick one and forge ahead.
 
Personally, I would not switch clock sources.  I would provide a constant, fixed timebase for internal logic, and design a path for bridging the input video signal to the internal timebase.  Typical implementations use either oversampling (and input pixel clock edge detection) or FIFOs.
 
-- Bob Elkind
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我最喜欢的恒定内部时钟方法是使用略高于最大输入像素时钟频率的频率。
然后在输入时钟和恒定时钟之间使用短FIFO（通常几乎为空）。
由于恒定时钟稍微快一些，如果始终在FIFO上启用写入和读取，则不会溢出，并且可以将非EMPTY条件用作FIFO下游的时钟使能或数据有效信号。
如果您的外部视频源可能会消失，您可能需要在检测到之后进入的视频时重置FIFO。
如果FIFO在外部源启动时由于时钟毛刺而变得“混乱”，这可能会有所帮助。
如果视频源本身有可用的时钟，我通常不赞成使用不相关的时钟进行过采样。
也可以使用FIFO的EMPTY输出来检测输入时钟频率。
在一段相当长的时间内计算非空循环可以为您提供合理准确的读数。
请注意，停止的输入时钟可以防止标志通过FIFO传播，但EMPTY标志是“保守的”，这意味着如果FIFO可能为空，只要您继续提供读时钟，该标志就会被置位。
因此，您还可以使用EMPTY循环计数检测缺少输入（频率小于某个阈值）。
-  Gabor



以下为原文

My favorite approach for a constant internal clock is to use a frequency that's slightly higher than the maximum input pixel clock rate.  Then use a short FIFO (it will be normally almost empty) between the input clock and the constant clock.  Since the constant clock is slightly faster, if you always enable both write and read on the FIFO you won't ever overflow and you can use the not EMPTY condition as a clock enable or data valid signal downstream of the FIFO.  If your external video source can go away, you'll probably need to reset the FIFO when you detect the video coming in afterwards.  That can help if the FIFO becomes "confused" due to clock glitches when the external source starts up.
 
I generally frown upon using oversampling with an unrelated clock if there is a clock available from the video source itself.  It's also possible to use the FIFO's EMPTY output to detect the incoming clock frequency.  Counting non-empty cycles over some reasonably large time period can give you a reasonable accurate reading.  Note that a stopped input clock can prevent flags from propagating through a FIFO, but the EMPTY flag is "conservative" meaning that if the FIFO could be empty the flag will be asserted as long as you continue to provide the read clock.  Thus you can also detect lack of input using the EMPTY cycle count (frequency less than some threshold).
-- Gabor