一直不清楚FPGA中Speed Grade的可选值的大小与FPGA速率是成正比还是反比关系，我对其不解。如下文中的提问者，所以今天GOOGLE了一下，找到了答案，如下英文。
简单的说FPGA的speed grade是一个相对标准，在现代版的（XILINX） FPGA 中，Speed Grade的值越大其速率越高，具体说详解下文（回答问题的2人均为Xilinx的员工）：
ASK：
Can anyone please explain what a speed grade is?
If i tell you my bicycle has a speed grade of 29, you’ll probably say something like: “Good for you.”, but you wouldn’t have a clue about what i just said.
So, can you explain to me how i should see the speed grades.
What do they stand for?
Who defines the speed grades?
Can i compare the speed grades between manufacturers?
I can’t seem to be able to find anything about the subject anywhere…
ANSWER 1：
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.
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.
Arthur
ANSWER 2：
As Arthur indicated, it is a relative term that is really dependent on the specific family:
-for CPLDs, it is generally pin-to-pin delays in nanoseconds (lower # = faster)
-for old Xilinx FPGAs (pre-Virtex), lower # was faster
-for modern (Virtex and later) FPGAs, the higher # is faster.
The speed grade influences a variety of timing paramters in the FPGA, including fabric (slice), multiplier/DSP48x, BlockRAM, I/O, and other resources parameters.
You really need to consult the specific datasheet to see specific details for timing based on associated speed grades.
For example, Virtex-4 speed grades are -10 (slowest), -11, and -12 (fatest)
Virtex-5 spede grades are -1 (slowest), -2, and -3 (fastest)
There is no correlation between these numbers. It is really a relative metric of performance within a specific family.
Cheers,
bt
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