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亲爱的大家,
我刚刚进入FPGA设计领域,并在最近几周做了很多阅读。 我相当自信我可以回答我自己的问题,但我只想确认一下。 场景: 我有一个24MHz的时钟 我通过IBUFG将其转换为内部信号 3.我立即使用DCM进行相位同步和PLL这24MHz高达300MHz的信号 我把这个300MHz的信号充电到我的逻辑 我的问题是: 在我的.UCF计时时序约束中,我只指定了对300MHz信号的约束,并假设它将通过DCM / PLL和引脚上升流。 有人可以确认我的简化时序描述的尝试是有效的(因为我对时序约束文档的解释是,只在一个CLK上执行此操作是有效的)。 干杯, 詹姆士 以上来自于谷歌翻译 以下为原文 Dear All, I am just getting into the realm of FPGA design and have done much reading in the recent weeks. I am fairly confident I can answer my own question but I just wanted to confirm. Scenario: 1. I have an incoming 24MHz clock 2. I translate this to an internal signal via an IBUFG 3. I immediately use a DCM to phase synchronize and PLL this 24MHz up to 300MHz signal 4. I BUFG this 300MHz signal to fan out to my logic My question is: In my .UCF timing constraints for clocking, I have ONLY specified the constraint on the 300MHz signal and am assuming that it will ripple up stream through the DCM/PLL and to the pin. Could somebody please confirm that my attempt at simplifying the timing description is valid (as my interpretation of the timing constraints documentation is that yes it is valid to do this on just the one CLK). Cheers, James |
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7个回答
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不,你有错误的方法。
在您的UCF中,您只需要限制INPUT时钟,24MHz。 这些工具非常智能,可以通过时钟管理和逻辑来抑制约束,而不是从逻辑返回到引脚。 顺便提一下,根据24MHz时钟的路由方式(即它是否会转到其他逻辑以及DCM?),您可能需要在IBUFG之后添加一个BUFG(是的,令人困惑的名称)。 如果它只从IBUFG路由到DCM,您可能不需要它。 问候, 霍华德 ----------“我们必须学会做的事情,我们从实践中学习。” - 亚里士多德 以上来自于谷歌翻译 以下为原文 No, you have it the wrong way around. In your UCF you should only need to constrain the INPUT clock, 24MHz. The tools are smart enough to ripple the constraint through the clock management and into the logic, not from the logic back to the pin. Incidentally, depending on how your 24MHz clock is routed (i.e. does it go to other logic as well as the DCM?), you may need to add a BUFG after the IBUFG (yes, confusing names). If it ONLY routes from the IBUFG to the DCM, you probably won't need it. Regards, Howard ---------- "That which we must learn to do, we learn by doing." - Aristotle |
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詹姆士,
通常,如果对输入时钟应用周期约束(在您的情况下为24 MHz),则通过连接的DCM将其传播到所有输出(作为所有这些时钟的新周期约束)。 你在使用什么部分,以及什么版本的软件? 报告的工具抖动是什么(由于输入时钟抖动,系统抖动和合成抖动,将24 MHz转换为300 MHz,从时间段带走了什么?) 我认为你的M和D值为M = 25,D = 2(从24得到300)。 Austin Lesea主要工程师Xilinx San Jose 以上来自于谷歌翻译 以下为原文 James, Generally, if one applies a period constraint to the input clock (24 MHz in your case) it is propagated through the connected DCM to all outputs (as a new period constraint for all those clocks). What part are you using, and what version of software? What is the reported jitter from the tools (what is being taken away from the time period because of input clock jitter, system jitter, and synthesis jitter -- converting the 24 MHz to 300 MHz)? I presume you have M=25, and D=2 for your M and D values (to get 300 from 24). Austin Lesea Principal Engineer Xilinx San Jose |
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感谢您的回复,我已经修改过它只考虑24 MHz时钟,它只通过IBUFG接口然后是BUFG(因为我在其他地方用于一些简单的任务)。很高兴我现在知道这些工具不会产生波动
回到DCM。我正在使用Xilinx ISE 13.4并正在开发Spartan 3E。 还不确定Jitter的规格吗? 这个设备上的PLL如此之高是一个问题吗?问候,詹姆斯 以上来自于谷歌翻译 以下为原文 Thanks for the responses, I have modified to have it only consider the 24 MHz clock, which only interfaces via an IBUFG and then a BUFG (as I use it elsewhere for some simple tasks). Glad I now know the tools don't ripple back through DCMs. I am using Xilinx ISE 13.4 and am working on a Spartan 3E. Not sure on Jitter specs yet? Is it a concern to PLL up so high on this device? Regards, James |
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当频率转换的乘法和除法常数差别很大时,我偶尔会看到工具和性能问题。
我不确定为什么会这样,但它可能是需要注意的。 ----------“我们必须学会做的事情,我们从实践中学习。” - 亚里士多德 以上来自于谷歌翻译 以下为原文 I have seen occasional tools and performance issues when the multiplication and divide constants for frequency conversion are considerably different. I am not sure why this is but it could be something to look out for. ---------- "That which we must learn to do, we learn by doing." - Aristotle |
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Ĵ,
3E没有PLL。 它有DCM。 完全不同的实施。 抖动和抖动非常重要。 抖动会导致它失锁。 对于较高的M / D值,CLKFX输出可能具有较大的抖动。 例如,不支持在频率合成模式(CLKFX-> CLKIN,CLKFX输出)中从第一DCM驱动第二DCM,原因是CLKFX上的抖动对于下一个DCM而言太大。 串联的两个DCM通常没有任何意义(不要这样做)。 Austin Lesea主要工程师Xilinx San Jose 以上来自于谷歌翻译 以下为原文 j, The 3E has no PLL. It has DCM's. Entirely different implementation. Jitter in, and jitter out, is very important. Jitter in will cause it to lose lock. For high values of M/D, the CLKFX output may have substantial jitter. For example, driving a second DCM from a first DCM with both in frequency sythesis mode (CLKFX->CLKIN, CLKFX out), is not supported for the very reason that the jitter out on CLKFX is too great for hte next DCM. Two DCM's in tandem generally never makes any sense (don't do it). Austin Lesea Principal Engineer Xilinx San Jose |
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谢谢奥斯汀,
很高兴知道谨慎的话。 我将回顾输入抖动规范,我假设我可以找到一个约束,允许我将它们添加到这个输入CLK上,这样它就会波及我的300MHz逻辑的时序分析。 最后一个时间问题。 我决定使用我用作CLKFB信号的CLK2X。 如果仅为我的CLKIN信号定义约束,您能否确认时序分析还将在时序分析中识别从CLKIN到CLK2X的这种关系。 干杯! 詹姆士 以上来自于谷歌翻译 以下为原文 Thanks Austin, good to know the words of caution. I'll review the input jitter specs and I'm assuming I can find a constraint that will allow me to add them onto this input CLK so that it will ripple through to my 300MHz logic's timing analysis. One last timing question. I have decided to use my CLK2X that I am using as my CLKFB signal. Could you just confirm that the timing analysis will also identify this relationship from CLKIN to CLK2X in timing analyses if a constraint is defined only for my CLKIN signal. Cheers! James |
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詹姆士,
是的,应该传递对输入的约束(并针对新的输出周期进行修改)。 Austin Lesea主要工程师Xilinx San Jose 以上来自于谷歌翻译 以下为原文 James, Yes, the constraint on the input should be passed through (and modified for the new output period). Austin Lesea Principal Engineer Xilinx San Jose |
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