Fitting a Real Time Clock to a Microbee Premium

***ChickenMan*** · 13-12-2021, 09:26 AM

The Premium mother board has facilities to fit a RTC using a MC146818, but also needed is a Ni-Cad battery which I prefer not to fit due to their long term leaking. I recently found https://silvestron.com/store/sb1287-rtc-replacement/ which looks like exactly whats needed. Has anyone fitted one of these to a Microbee Premium and if so, what other mods to the board would be needed to fit one. Another alternative would be to use a Dallas DS12887 and do some surgery to bypass the flat battery they contain and add a button battery on top as seen Youtube vids on that.

Any help appreciated.

CheshireNoir · 13-12-2021, 10:30 AM

Not sure it's useful, but I did see that Necroware had designed a "drop in" replacement for many of the RTC modules out there fitted with a cell on the module itself. It's quite a neat design! It's GPL3 so an excellent place for remixing from.

https://github.com/necroware/nwX287

https://www.youtube.com/watch?v=svPNxILeQEw

Cheers!

Chesh

***ChickenMan*** · 13-12-2021, 12:09 PM

Thanks Chesh for that, looks similar to the Silvestron one above but would work out much the same price as above but a lot of work obtaining parts, assembly, etc.

Or would a simpler solution be to use the original MC146818 RTC but instead of the 3.6v Ni-Cad battery, relace it with a 3.0v button battery. Would the 3.6v ver 3.0v make any difference or would the clock run slow ?

Ernest · 13-12-2021, 06:47 PM

If a system already has a functioning MC1481818 fitted, the best solution would be to replace the old NiCad with a Ni-MH, small 3.6V pak's are prolific for use in solar garden lights.
I would steer clear of button cells for this chip as it is a lot more power hungry than the latter DS12887 which has internal clock circuitry and internal Lithium battery.

If you need to replace the MC1481818 with a DS12887A or add clock function to a Premium Microbee that has none, then there are a couple of things you need to watch out for. It has been quite a few years since I did one but as best I can tell from my notes.

1) The DS12887A is configurable for either Motorola or Intel timing - we need it to be configured for Motorola this is done by holding pin 1 at Logic high. Pin 1 on the Premium it connected to the +5V rail, so we need to cut the track at IC18 (RTC) pin 1 on the top side between CV2 and the RTC. Connect a jumper from Pin 1 to Pin 24.
2) Remove all the Clock components from pins 2 & 3 - CV2, C7, R3, G1 &R4 absolutely necessary C8 may as well while at it. Also remove R14, C6 D1, D2, R7 & R1, Cut the track to Pin 20.
3) We now need to provide isolated +5V power tp Pin 24 of the RTC so the internal battery doesn't flatten over night while trying to run the Microbee. Use a Schottky diode from a convenient 5V to Pin 24, I fitted the anode end of mine in the cathode pad of D1 +5V and the cathode end in the cathode end of D2.
4 There is a vague reference to a jumper to restore +5V power to IC15 - maybe a track gets interrupted when we cut power to pin 22 of the RTC, not 100% sure there.

Ernest

***ChickenMan*** · 13-12-2021, 07:48 PM

Thank Ernest, will give it a go, though with Christmas coming things are getting somewhat busy here.

someone · 13-12-2021, 08:10 PM

Most DS12887x's must be 20+ years old with expired batteries thus someone threw all bar one of theirs out thinking they'd never be used again.
Many old IBM PC motherboards used them or equivalents.

***ChickenMan*** · 13-12-2021, 08:22 PM

A nice video on Youtube shows how relatively simple it is add a button battery to a DS12887

CheshireNoir · 13-12-2021, 10:17 PM

(13-12-2021, 08:22 PM)ChickenMan Wrote: A nice video on Youtube shows how relatively simple it is add a button battery to a DS12887

Haha! Necroware again! The same guy who made the project I suggested :-D
Chesh

Ernest · (This post was last modified: 14-12-2021, 07:49 PM by Ernest.)

I can vouch for this myself, purchased a couple of ebay and as someone points out, the internal lithium batteries were flat.
Took to it with a Dremel and both were up and running.
The first one was used on an external to the Mainboard RTC adapter board and I mounted the button cell onto the module.
The second one was for a Premium Mainboard so didn't have the clearance required, so brought the internal line that originally went to the +v connection of the internal battery, out to Pin 20 of the module. Then cutting the track from the pin of Pin 20 on the board. With a little "outside the box" thinking I was able to add a strategically placed mounting pin hole in the Mainboard and combined with the original NiCad battery negative pin hole mount, a button cell holder and run a short wire between the +v terminal of the holder to Pin 20 of the RTC.
This battery has successfully maintained the correct time now for about three years, with it's placement just behind the RGB video output D9 connector, I can replace the battery without even removing the Coreboard.

My Note #4 in the post #4 above, I think now the loss of power I referred to was the cutting of the power for Pin 20 rather than pin 22. If anyone decides to have a closer look at this, let us know what you see in relation to this.

Ernest