[Coco] Battery-Backed RTCs for CoCo?

[Gene Heskett]

On Tuesday 13 November 2018 10:54:01 Mark J. Blair wrote:

> Thanks for the info, folks! I'm daydreaming about a cartridge project,
> and I was wondering what RTCs were used and preferred in the old days.
> If practical, I might want to make something compatible with old
> software. The DS1315 as used in the likes of SuperIDE and TC^3 is
> still available, but it's quite expensive. So, if I can't find a
> cheaper way to achieve compatibility with it, I may break
> compatibility at the cost of having to write new code to work with it.
> It's not a question of the price of one chip, which I can easily
> afford... it's the engineer in me, who sees a $17 clock chip and
> dozens of < $1 ones, and can't bring himself to design in the $17 one.
>
> --
> Mark J. Blair, NF6X <nf6x at nf6x.net>
> http://www.nf6x.net/

You'll have to research the dollar ones, Mark. Anything that has to have 
an access integer password keyed in bitbanger style just to read the 
time, will NOT be compatible with any other serial com protocol, 
something I learned with the B&B XT-RTC

I don't think I ever published it, but I wound up useing a fully software 
clock, essentially the same as the software clock in my os9 bootfiles 
except it did a settime only at powerup. I think I wound up doing a 5 
tick second every 10 minutes, which kept it closer to walltime over long 
bootup sessions. It will of course depend on the accuracy of the 3.58 
crystal the coco's clock runs on. They weren't even in the same ballpark 
as the crystal in the average color tv of the day since those are 
required to be phase lockable to the NTSC signal which has a +-10 hz 
tolerance at 3.5795454545454 megahertz which you get from a Rhubidium 
$tandard. 

Interestingly, because of the doppler effect on the relay satellites, 
which they didn't waste fuel keeping them precisely in the center of 
their "box" 22,300 miles out, were allowed to do a figure 8 in location 
as long as they stayed within their "box", I've seem drifts approaching 
100 hz/second over a 24 period, just from the satellites orbital 
motions. The FCC calmly looked the other way when we genlocked the 
station clocks to the incoming satellite signal because fixing that 
would have used up the birds station keeping fuel in less than a year.

And now, you know "The rest of the story." :)

Something without any access preventer, like what Tony used in Tony's 
4n1, is out because powerdown and powerup noises will reset its time to 
random values about every other power cycle.  The one in the TC^3 
doesn't seem to suffer from either problem, but surely those ease of 
access and power cycle noise immunity features are available in one of 
the dollar chips today, but may need lower vcc volts than our common 5 
volts, so there may be an interfacing problem. You may be able to enter 
the specs into digi-keys search engine and get a list of chips to at 
least DL the pdf's to see if one of its hits is usefull.

Oh, and it should run several years on the PC's std CR-2032 battery.
Something my garage door buttons can't do, they seem to go away in 18 
months or so. So add the cost of one of those sockets and a battery to 
the B.O.M.
-- 
Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>