Hi > On Jul 2, 2020, at 5:30 PM, jimlux <[email protected]> wrote: > > On 7/2/20 11:37 AM, ed breya wrote: >> It's been fun reminiscing about all these dividers and techniques, but >> getting back to the OP, the original search was for a divide by 5 with "low >> power" and operation from 5 to possibly 3.3V, and clocking properly at 50 >> MHz. One would assume also minimal size and complexity, and low cost. > > You forgot to add rad-hard.. I was the OP - This has been a fascinating thing > - we have a breadboard that uses a fancy clock distribution chip that > consumes close to a watt (and has too much jitter, as well).. > > > > I was thinking on the long commute to (tele)work from downstairs to upstairs > (or maybe on the commute home going the other way).. You know, we don't need > all that functionality in the clock chip, there must be a way to do a divide > by 5, and that has good noise properties - 2 and 4 are trivially easy (I > thought), there's probably some easy way to hook up 3 (or 4) flipflops and > get a nice divide by 5, and maybe even 50/50 duty cycle out. > > So I posted the question - because I've seen discussion of good divider > designs here, and I was sure that someone would come with a novel suggestion. > > What I have learned is that > > 1) All those clever handbook designs and data sheets that I grew up with in > the 70s,80s, and 90s are just the ticket, but you can't actually get the SSI > MSI parts any more. > > 2) One can brute force design simple functions by just trying all possible > connections and see if it works. What a clever idea! All that work with > Karnaugh maps, etc. trying to come up with minimalist designs, and you can > let your idiot savant assistant (the computer) just grind through all > possible designs. Of course, now that you have that clever efficient > design, because of #1, you can't build it. > > 3) It's hard to even find programmable logic that is simple and small. All > the mfrs tout their latest tiny parts with *only half a million gates* (I > exaggerate, but you get the picture)
If they want to sell you a fully self contained "million gate" device for a couple bucks, is that really a bad thing? Sure, if it’s in a thousand pin BGA, it’s a bad thing. If it’s in a < 40 pin package that you can get on a small board …. maybe not so much. Bob > > 4) What I'd be happy to do for a room temperature breadboard probably won't > work over temperature - and an "existence proof" that it can be done at room > temp doesn't mean you can find parts to build it to work over temp (See #1, > again) > > > > >> One piece of info that is missing, is how many of these are needed. If it's >> a one-off situation, that's way different from mass production and assured >> parts availability. If only one or a few are needed, then I'd contend that >> the 74AC390 is the way to go. It definitely will work at the lower end of >> the supply range, and is the simplest in parts count - one piece, and no >> doubts about external prop delays that would be associated with getting >> other types to divide by 5. > > > >> If there are truly lots of NOS ones out there, it should possible to just >> buy a bunch for all anticipated needs. I can't imagine they would be very >> expensive, except for the issues of volume versus transaction cost. I've >> never tried to buy old parts from these kinds of distributors, but I would >> imagine there would be minimum order requirements or fixed cost. So, getting >> one piece might cost $100, while getting a hundred pieces may be $110, and >> so on. There's no harm in asking and negotiating. > > We *are* building the eventual system to fly, and yeah, we've got tubes and > tubes of old ICs at work (JPL) or, as Rick mentioned, there's always > Rochester Electronics, who have a warehouse full of old wafers and dice. But > I'd rather not. > > For those who come after me, and are perusing the archives (thank you google) > - here's some reasons why old parts are a pain (and a curse). > > 1) Reliability people freak out about packages that have not been kept in > absolutely pristine conditions with a full paperwork trail of certifications. > The humidity might have gone up. Oxygen or Helium might have leaked in. There > might have been latent ESD damage. So you'd have a tube of parts with date > codes from the 80s or 90s that *work* (over temp, etc.), but the mission > assurance folks will want a bunch of them to do destructive analysis. Making > sure there's no latent degradation, etc. That can cost a lot. > > 2) You CAN get parts from Rochester, and they're freshly packaged, from known > good dice, etc. That's not cheap either. But, is probably cheaper than #1. > > 3) the biggest reason - There are innumerable cases where someone used "end > of life" or "flight spare" parts, just this once. And then, the next mission > comes along and wants to do a "build to print" to claim heritage - and you > spend a lot of time trying to track down NOS parts, or in design reviews > trying to say "well even though all the parts are different, except the > resistors, it really isn't a new design". > > We used the Xilinx Virtex II in several of our radios that have flown to Mars > (in the Electra UHF radio on all the rovers since 2003, for instance). I > think we have the largest stock of flight qualified Virtex IIs in the world, > because people still want to use Electra radios, as a "build to print". > This is not healthy. One has to have development systems to test software, > one has to have spare units, etc. All consuming those few remaining > XQR2V3000's in the world. And one has to have the tool chain as well (Long > since obsoleted by Xilinx, and doesn't run on any "new" versions of Windows). > > As a project manager I get the desire to use something that is known - you > know what it will cost, you have the procedures, there's very little > uncertainty in the cost and schedule, which is what I care about as PM. > > As a design engineer, I despair of having to support a design that is 20 > years old. I never know whether the next one is the one that fails in test or > otherwise not work, and there's no "fix-it" parts, and it's going to be my > job to tell the management - uhh, you're gonna need a different radio. > > > --- > > Back to divide by 5 - If I can offer a suggestion to the design team, they > can run with it, look for parts that are reasonably available, and feel > comfortable that when somebody says "let's do a build to print" in 10 years, > they'll still be able to get the parts. > > > Now, for myself, making something with vacuum tubes is something I've not > done since the early 70s. I am intrigued.. I grew up in the 60s reading > Millman and Taub (the big yellow book) as a child (EE professor as a parent), > I read about all the vacuum tube circuits, but what I built were transistors. > My parents were no fools and not about to let me work with a B+ supply of > 100V. Those 2N404s and 2N1613s were what I used. And some early UJT, but I > can't remember the number. And the RTL digital logic parts like the uL914 > dual 2 input nand. > > But I never went back to the vacuum tube designs.. Ah, one of those things > for retirement, perhaps. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
