[M100] REXCPM battery

[Brian White]

Took some measurements

This is with the BAT54C diodes.

While not installed in 100:
BATT  2.99v
RAM_RST  2.91v 7uA
SRAM VCC  2.76v


While installed in 100, power on, wall power, original tandy 6v adapter:
RAM_RST  4.85v
BATT, installed  3.0v
BATT, not installed  3.65v 0.5uA



bkw

Re: [M100] rexcpm battery

[Ken St. Cyr]

Yeah, that's one of the disadvantages of using schottkys - they do tend to leak 
more reverse current. I appreciate the offer to send me a BAV170. I regularly 
put in orders at Mouser, so I'll just add some to my cart when I do my next 
order. Very kind to offer, though 🙂

//Ken

From: M100  on behalf of Brian K. White 

Sent: Thursday, September 21, 2023 10:53 PM
To: m100@lists.bitchin100.com 
Subject: Re: [M100] rexcpm battery

I've been reading about charging protection for lithium coin cells, and
I think we might want to swap out the BAT54C for something with as
little reverse leakage as possible.

I thought a few uA wouldn't really matter, especially if only during
on/active time, and didn't worry about it, but I guess even that causes
the cells to die early and only last a few months instead of years.

So I think you should not use the BAT54C from the original BOM and use
maybe BAV170 instead:
https://www.digikey.com/en/products/detail/diodes-incorporated/BAV170-7-F/822819

or any of these:
https://www.digikey.com/short/q9bfjch5

This was my other pick:
https://www.digikey.com/en/products/detail/central-semiconductor-corp/CMPD6001C-TR-PBFREE/5325257
Lowest reverse leakage but a little higher forward voltage drop and also
more expensive.
Probably neither difference actually matters much at the voltages and
currents involved.

Maybe the BAT54C is actually fine as long as the computer isn't turned
on for long periods in 90F heat.

The reverse leakage graph for the BAT54C at 2v (difference between 5v
from the computer and 3v from the battery) at 25C (77F) is only 0.3uA,
but could go up around 5uA for 3v (say the batts are almost dead at 2v,
so 5-2) at 40C (104F).

So... maybe not really a problem 99% of the time. But still.

I'm ordering some for myself and if you mail me your address I'll mail
you some so you don't have to make a whole digikey order and shipping
just for that.

No change to the PCB other than the silkscreen. The new part drops in
the same place.

Sorry about that.

--
bkw

On 9/20/23 14:19, Ken St. Cyr wrote:
> It turned out nice and looks pretty solid (at least in the pictures). I
> ordered boards and parts the other day, so I'm just waiting for those to
> arrive now. I'll report back and share my experience after getting one
> together -
>
> //Ken
> 
> *From:* M100  on behalf of Brian K.
> White 
> *Sent:* Tuesday, September 19, 2023 10:18 PM
> *To:* m100@lists.bitchin100.com 
> *Subject:* Re: [M100] rexcpm battery
> I have repaired my REXCPM and now the github readme includes pics that
> show a good way to mount a qwiic connector and where to connect bodge
> wires if you want to take on the challenge of such small loose wires
> going to such small places. All 4 solder points are at least a
> reasonable chip leg, no soldering directly to delicate traces. So the
> wires are reasonably robust in the end. You can install & remove the
> rexcpm module without straining the wires. I will probably further
> secure them with glue sometime later but haven't yet.
>
> I don't think it's worth it just to get the connector if your original
> pins are still intact, even though the connector is more convenient in
> the end, but since my original pins rotated and broke their traces
> anyway, I had to do a bodge wire repair anyway. And mounting the
> connector seems to be solid enough by super-glueing it to the top of a
> chip, and by using one of the black versions of the connector which I
> think is a material that the glue can stick to easier. The natural
> colored ones look like they might be something no glue will stick to.
> The black ones feel like they have some glass filler, and can be scuffed
> to a rough texture. Seems to be pretty solid so far though I'm not
> testing-to-destruction just to find out. :)
> single https://www.sparkfun.com/products/14417
> 
> 10 pack https://www.adafruit.com/product/4208
> 
>
> My rexcpm is running and working so I can resume testing battery life.
>
> I think it's safe to order pcbs if you want now. The dupont breakout
> cable is a good solution for keeping the REXCPM side of things simple
> and stock, and adding the single wire for gnd is easy, reasonably safe
> against blunders, and leaves the REXCPM still compatible with it's
> original bus board and cable.  So the bus board can keep the qwiic
> connector.
>
> I have tested the 102/200 board now too. good to go. I don't have a
> pcbway or gerbers up for that yet but will shortly.
>
> --
> bkw
>
>
> On 9/19/23 04:29, Brian K. White wrote:
>> My boards came in and at least the model 100 one works fine.
>>
>> There's room, but no good way to solder the qwiic connector to the
>> REXCPM, so the most practical way to handle the REXCPM side is just
>> use the pre-made adapter cable that has female dupont sockets, and
>> stick them r