An alternative approach to CMOS/RTC/Battery replacements?

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simonm
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An alternative approach to CMOS/RTC/Battery replacements?

Post by simonm » Mon Sep 14, 2020 11:29 am

I've got an A5000 with a poorly CMOS RTC on my workstation at the moment, so I'm considering my repair strategy. I've read with interest the various threads and challenges around replacing the chip - lost pads/continuity checks/battery replacements etc.

I also read Retrokit's article about making a daughterboard, and it occured to me that there's a lot of very cheap prefab RTC modules (search for PCF8563 boards) around these days what with RPi and Arduino hobbyists using them. eg. https://www.ebay.co.uk/itm/233424562323
s-l1600.jpg
Having looked at the RTC circuit schematics for the A5000, these boards seem to provide the same functionality except that they carry the SMD PCF8563T chips (which dont have the 240 bytes CMOS RAM) whereas we need the PCF8583T variant (which do).

So I'm thinking something like:
- Desoldering the 8563T on the module, and drop-in replacing it with an 8583T (because easier to desolder on a separate board and minimise risk of losing pads/further damaging main Arc mobo)
- Use the 3.3V battery already onboard the module, which saves having to fit a new battery on the Arc mobo, plus it keeps battery separate in case of future leakage - 2 for 1!
- Make modifications to bypass main Arc mobo RTC/battery circuit, (not sure what modifications yet, cut out old RTC chip rather than desolder?) and attach the daughter board instead: +5v, 0V, SDA, SCL connections as per Retrokits solution.

Since I'm no whiz at electronics, I thought I'd check with the stardot brains to see if I am missing anything fundamental here?
Possible issues I can think of:
- The pinouts for 8583 and 8563 are slightly different : Pin3 on 8563 is INT, on 8583 it's A0, Pin7 on 8563 is CLKOUT, on 8583 its INT. This might be the showstopper, but the A5000 schematic shows these pins are either NC or 0V. We can perhaps wire accordingly using the header on the board.
- The 8583 consumes more standby power than the 8563, (2mA@1.0V vs 250nA@VDD=1V) so the 3.3V non rechargable cell on the module might not last as long.
- I'm assuming the existing I2C signals from Arc to this module will work fine as long as theres an 8583 in the circuit
- A neat way to mount the module firmly & tidily on the existing Arc mobo

Is this a workable idea?

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flaxcottage
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Re: An alternative approach to CMOS/RTC/Battery replacements?

Post by flaxcottage » Mon Sep 14, 2020 2:59 pm

Sounds great to me but then what do I know. :lol:
- John

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DaveLecky
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Re: An alternative approach to CMOS/RTC/Battery replacements?

Post by DaveLecky » Tue Sep 15, 2020 9:22 am

Hi there,

I had the exact same thoughts with my Riscpc last month, I bought a couple of boards but the chips I had were too big to go onto the board, so put the idea to one side and built a couple of rough prototypes which worked a treat.

I played with a couple of pcb design apps but could not get them to play ball, I wanted to generate Gerber files to send to a pcb manufacturer to build them properly, just for the hell of it, just couldn’t get my head round the software.

This solution was much easier than trying to fit and resolder tiny surface mount components on the motherboard, connections can be picked up elsewhere on the mobo if the battery area is badly damaged.

The final builds were encapsulated in heat shrink sleeving.
The battery module has diode and resistor built in.

C6878CD3-5854-477A-8852-D82E109C1582.jpeg

Here is another attempt...Built onto a cr3023 battery holder.....I missed out a resistor on the negative side of the battery though.. :oops:

638720C9-2704-4E98-9C6F-12FAB5DEB296.jpeg

This is the first project I’ve done like this so don’t be too critical on my bad workmanship...it can only get better... :wink:


I look forward to seeing your progress on the prebuilt module....it would be great if it works.

Dave
Electron Plus 3 and plus 1, AP6 and Home made MMFS PP SD interface
Electron Plus 1 Pres AP3/4
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simonm
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Re: An alternative approach to CMOS/RTC/Battery replacements?

Post by simonm » Tue Sep 15, 2020 1:26 pm

Interesting to see your wip Dave. You're braver than me, I didn't feel any urge to build my own at all; not least because I dont have any individual components!
It's also good to hear a similar solution will work for RiscPC's as I have one of those too, albeit with a missing battery but working RTC. I feel like refitting a new Varta mempac is just a time extend before leaky battery happens again. A better alternative is a separate battery box with eneloops I guess.

Since I posted this it's come to my attention that CJE sell a ready made podule solution, which may be of interest to some. http://www.cjemicros.co.uk/micros/indiv ... CJE-RTCPOD

Anyway, I've decided to give this a whirl, and have ordered 10x 8583's and a couple of these modules, so I'll update when I have them (coming from China so will be a while yet).

Out of interest Dave - where did you attach your daughterboard to on the main mobo?
I'd love to end up with a cheap reliable solution that can be neatly fitted with minimum physical changes to the mother board.

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DaveLecky
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Re: An alternative approach to CMOS/RTC/Battery replacements?

Post by DaveLecky » Tue Sep 15, 2020 3:45 pm

Hi Simonm,
I was aware of CJE micros modules, £30, this is why I went the home made way.

Here is my effort mounted on the back of the backplane for testing.

E226A02A-CCD6-4C9B-ACA9-EB3A3DDC0FC0.jpeg

I would still like to have a couple of proper pcb’s made up, I’ll have to try again with the software.

Maybe someone has already done this and has the files to share.... :D


Dave
Electron Plus 3 and plus 1, AP6 and Home made MMFS PP SD interface
Electron Plus 1 Pres AP3/4
Beeb issue 7 with IFEL 16 socket Rom/Ram board
Master 128, BooBip multi OS,IFEL SD card
StrongA RPC, A3020
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