[time-nuts] low power divide by 5

[jimlux]

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)

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.