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FPGA constraint设置讨论

菜鸟
2010-08-26 14:06:36     打赏
FPGA设置开发中有很多难点,其中之一就是constraint的设置。 望各位朋友展开讨论,并给予指导。 感谢!   当我们在设置FPGA constraint的时候,我们应该是首先要了解该器件的参数。如频率参数,setup time和hold time参数等。 但问题是,有如此多的参数,我们该使用datasheet中的哪些参数来估算PERIOD/OFFSET的 time scale? 如何设置FF—FF之间的setup/hold time?是否是使用FROM_TO的TS_C2S进行设置?       还有问题的是,比如为了保证得到正确的传输,DATA_IN 的offset该如何设置?   如下例子,我看到xilinx 介绍说DATA_IN至少要比CLK早20ns到达,问题来了:我们知道对于已经设计好的device,DATA到越早越好,但问题是对于使用DATA_IN_BEFORE而言DATA到的时间已经给定了,不能改变。    也就是说我们只是在开发接收端的功能,这时Tsu,Tclk已知,其实Tdata才是使用到constraint的设置后强行要求满足的参数,(Tin_before=Tdata+Tsu-Tclk;=> Tdata=Tin_before+Tclk-Tsu),在Tin_before刚好等于Tdata+Tsu-Tclk时,是正确latch到data的极限,为了保证正确latch到数据那么这个时候 应该把Tin_before设置小一点才能得到更小的Tdata,也就是DATA才更早到达FF.可为什么xilinx的constraint说明却要写成Tin_before>=Tdata+Tsu-Tclk ,如果没有仔细分析直接去一个比较大的值是否会造成Tdata比较大,造成latch的错误?             以上是本人的观点,请各位给予指导,谢谢           



关键词: constraint     设置     讨论    

菜鸟
2010-08-26 16:18:09     打赏
2楼
    I read the xilinx’s constraint guide, it said that Tin_before>=Tdata+Tsu-Tclk,      we knew the Tin_before is good as large as possible for a fixed device that we can do nothing.       the question is that the designing FPGA device receives the data from other device when we use the DATA_IN_BEFORE to set the constraints,so the Tin_before is a known characteristic,we can get it using the oscilloscope. Using Tin_before to set the Tdata is our purpose,so optionally choosing a large Tin_before that is larger than Tdata+Tsu-Tclk maybe get larger Tdata,result in failure for latching data.my guess is that setting small Tin_before characteristic can get small Tdata,so can latch correct data.       So is the constraint book wrong, or my thought wrong ?   When we want to set the constraint, how do we estimate the period,DATA_IN_BEFORE,DATA_OUT_AFTER?   Which characteristics do we need to get from the FPGA datasheet ?    And,when we set the global constraints,because have IOB’s setup and CLB’s setuptime,which characteristic do we choose ?   How to set the FF ot FF timing,like setup/hold time ? use FROM_TO’s TS_C2S ?    And,because both IOB and CLB have FFs, their FFs setup/hold time characteristics are different, so how to estimate the FFs’s timing ?          Expect the reply,thanks everybody    

菜鸟
2010-08-26 16:25:26     打赏
3楼

以上问题我用英文写了一份到国外论坛求教,描述可能清楚一点。论坛好像不能付附件,可惜


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