How to Read a Data Sheet
Often, the best ideas for my blog postings come from what I am seeing and experiencing on the forums, or from the forum participants themselves.
This blog is from one of those postings, “how do you read the data sheet?”
What is important? What does it all mean? Where do I start?
This will probably turn into a multi-episode set of posts, so let’s get started.
Hey, that is not the First Document!
OK, so I cheat. I am going to start with what we used to refer to as “section 3,” the DC and Switching Characteristics. Why? The first section is the overview, and should not be the subject of controversy, or have issues or problems. If it does, let me know.
The document linked above is all about the proper ‘care and feeding’ and potential performance of the devices in a particular family. This example shows the new 7 Series 28 nanometer family, specifically, the Virtex-7 device family.
First Things First
Table 1 lists the absolute maximum ratings. These are the voltages and temperatures where you need to just “not go there.” Xilinx has always adopted a stricter interpretation of what is a maximum condition than its competitors; these are values that if you operate below, or even at them, the device’s lifetime is unchanged (no damage occurs). Thus, if you go beyond these values, you may (or may not) experience a shorter operating life.
I am often asked if it is OK that the I/O pin voltage overshoot is some value, say 4.0 volts. Well, this section says it should be less than 3.3 + 0.5 volts, or 3.8 volts. No, operation at 4.0 volts is bad; do not do it! Will it fail immediately? Most probably not. When will it fail? Sooner than it would otherwise, and certainly sooner than the 15 year typical lifetime that we claim in our product qualification reports.
Second Things Second
Table 2 lists the recommended operating conditions. If the device operates within these limits, then everything that follows in the data sheet is assured. If you operate outside of these limits, then there is no guarantee of anything. Again, stay within these limits. If you go outside of these limits and have a problem, the first thing a Xilinx support person will ask you to do is to operate within the limits, and then call them back if there is a problem.
Will it ‘work’ outside of these limits? Probably it will. But, we characterize everything inside those limits, so we can only guarantee what we characterize and what we test.
At the time of this blog’s writing, this table was empty. No numbers! This tells you we have not finished characterizing the family yet. Not to be alarmed, as you may go back to a previous family, or a similar family (like Kintex-7 FPGA) and see what kind of numbers might appear after characterization is complete.
Join us Next Time
I will stop here with this post, and I will continue it later. Probably one of the most mysterious elements of the data sheets is the relationship between the numbers in the data sheet and the speeds files which describe the timings which are built into the software. This will be my subject next time.
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