===============================FPGA的速度等级（speed grade）（1）
XILINX公司FPGASpartan 3E系列XC3S500E速度等级为4.但一直不知道是什么意思.
通过学习知道,
(1)CPLD与FPGA的速度等级定义的区别
(2)不同的公司FPGA的速度等级
(3)同一个公司的不同时期的定义也是不一样的,如XLINX公司
具体内容可以参考以下材料.也可以在GOOGLE里输入 FPGA SPEED GRADE
(4)对于xilinx公司的FPGA的速度等级定义，个人观点：它不是以前所定义的其内部的一些逻辑单元信号传播所花的时间，而是一类内部逻辑器件的运行速度，这些逻辑器件运行时满足一定要求的时钟频率．它没有具体的物理的数据意义，只是一类内部逻辑器件的运行速度的的代号．
There is no consistent definition of a speed grade for all devices. Even for Xilinx,
speed grades mean different things depending on if you are referring to a FPGA or a CPLD. For CPLDs, speed grades represent the time it takes for logic to go through the device (eg. in <= out). So a -10 device means that the device is guaranteed to send a signal from an input pin thru to an output pin in under 10 nS. So for CPLDs, the lower the number, the faster the part is. This is standard for CPLDs across all vendors so this can be used for device comparison purposes.
what is the valve of standard grade(FPGA)
However for FPGAs, they don't use the same definition for speed grade. Originally speed grades for FPGAs represented the time through a look up table but now the speed grade doesn't actually repesent a timing path. I am not sure if it is the same for other vendors, but for Xilinx FPGAs higher numbers are faster. Each speed grade increment is ~15% faster than the one before it. So a -5 is 10% faster than a -4 speed grade. Determining the speed grade of Xilinx devices Q: I am having some trouble understanding the numbering system Xilinx uses for speed grade. Could someone explain what the numbers are and how to tell which speed grade is installed on my XS40. FWIW I have an XC4010XL FPGA. The available speed grade choices are -1, -2 -3, or -09. A: You usually see the speed grade imprinted on the chip on a line by itself. For an XC4000, you might see " 3C" printed on the chip. That means the chip has speed grade -3. (I think the "C" stands for commercial temperature range.) For XC4000 devices, the number is roughly equivalent to the propagation delay through a CLB. So a -3 speed grade implies 3 ns of delay through a level of logic, -2 means 2 ns. Don't be mislead by -09, it actually means 0.9 ns of delay. The situation changes with Spartan and Virtex devices. Now a larger number means the device is faster. Reference : http://www.xess.com/faq/M0000236.HTM
转自：http://blog.sina.com.cn/s/blog_5e9a27140100c742.html
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FPGA的速度等级（speed grade）（2）
Question: From: vlsigeek
Date Posted : 12/11/2004 4:42:49 AM Hi guys,
What is the speed grade in FPGA. What it tells actually.
Thanks in advance,
Soundar Comments: From: vlsi_giant
Posted : 12/27/2004 12:54:29 AM Hi
It is actually the min I/O delay for that device.
Ex: for altera MAX device EPM7128... -15
this -15 indicates this device has min of 15 ns. i/o delay.
yogesh | 
