Freescale replied with a link to a Kinetis data sheet. Section 3.8 reads:
A typical value is a specified value for a technical characteristic that:
- Lies within the range of values specified by the operating behavior
- Given the typical manufacturing process, is representative of that characteristic during operation when you meet the typical-value conditions or other specified conditions
Given that typical is "representative of that characteristic" one might assume this means one can expect the part to behave much like the specified typ data. But the example shown on that datasheet just a few lines later rates a current with a min of 10, typ of 70, and max of 130 µA. The standard deviation is enormous. The typ value teaches nothing about the range of values to expect (though, in that example, the min and max tell us everything we need to know; wise engineers will create a design that works properly over that entire range).
Three weeks after I asked NXP replied: “We have not seen a universal definition of ‘typical', though it has been one of the most commonly used terms in the industry and product. One interpretation would be that ‘typical' represents 'material that comes from our production process under standard SPC (statistical process control.'"
Again, this is interesting and useless, at least for practicing design engineers.
Atmel never responded.
The bottom line is that the meaning of a "typical" spec is devoid of engineering value. If a spec isn't guaranteed, it's not a spec -- it's an aspiration.
What's your take? How do you use typical specifications? And what do you do when a spec has only a typ, and not min and/or max, value?
Jack G. Ganssle is a lecturer and consultant on embedded development issues. He conducts seminars on embedded systems and helps companies with their embedded challenges, and works as an expert witness on embedded issues. Contact him at firstname.lastname@example.org. His website is www.ganssle.com.
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