Hi The PDP-8 had so much code that depended on un-documented instructions that they had to include them in later versions of the machine....
Bob On Aug 14, 2010, at 12:01 PM, "J. Forster" <j...@quik.com> wrote: > I think you missinterpret what I meant. Two examples: > > I've seen programmers who use "instructions" that are not part of a uP > instruction set and are undocumented, just to be "clever". If a different > brand of chip, or even a different rev., the chip does something > completely different. These guys should be strung up by their tender > parts. > > I've also seen transistors used as avalanche switches (basically a failure > mode). If a different production run has improved normal mode performance, > the avalanche function may vanish. > > FWIW, > > -John > > =============== > > > >> J. Forster wrote: >>> FWIW, IMO any engineer who uses undocumented or uncontrolled parameters >>> or >>> instructions in a production design is a fool. >>> >>> If you are that silly, you must fully specify the selection criteria. >>> >>> -John >>> >> >> Or, has their back against the wall and can't do it any other way. >> >> How is this any different than using trimpots or hand select? >> >> >> For years, folks have hand selected matched pairs of devices, since the >> circuit requires tighter tolerances than the mfr guarantees. >> >> Many, many RF designs have "select at test" pads to set levels or tuning >> stubs depending on what the actual gain or impedance properties of the >> active devices are, or for trimming temperature dependencies. >> >> >> Would you say that the engineer is a fool for not just specifying >> tighter tolerances.. the tighter tolerances may not be available from >> the mfr (who has to respond to many customers, most of which will be >> happy with the standard performance). It's sort of a tradeoff.. do you >> go to the mfr and say, I need a better grade of part, or do you buy the >> run-of-the-mill part, and sort them. >> >> You might decide to do the latter for competitive reasons, e.g. rather >> than the mfr producing a better grade of part, and potentially selling >> it to your competitors too, you keep the "secret sauce" in house. >> (Granted you could have the mfr make/select a proprietary part for you.. >> that's basically changing who does the work, but doesn't change the >> underlying design) >> >> Even manufacturers do this, for instance with speed grades on things >> like microprocessors. They don't have enough process control to >> guarantee a particular speed, so they make em all, and then sort them. >> >> >> The other thing is that the selection criteria might not be knowable in >> a standalone sense. That is, you have to put the part into the circuit >> and see if it works, rather than measuring some device parameter. I >> would agree that to a certain extent, this implies that you don't really >> know how the circuit works, but it might also be that the most cost >> effective approach is to use empiricism, rather than analysis. >> >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.