Yes to most of your understanding, not yet on the 4-cell test.
I just ordered the pcb's yesterday so it'll be about a week.
Although there is not much to actually test about the updated board.
It's just more of the same. It's the same circuit. All it changes is
adding more celles and a ground wire.
The gnd wire isn't really a new addition because when the boards are
installed in the machine, there is an equivalent gnd connection through
the normal gnd pins in both sockets.
Both boards do have their local GND connected to the GND pins on their
own respective sockets, and both sockets are connected to the same GND
rail all throughout the 100. So when both boards are installed, there is
a common GND connection between the two boards the same as a wire.
Nothing about the fitment changes, it's the same pin headers and same
battery holders. The pin headers are pretty tall at just over 4mm and
the was a real question if that would fit or if it needed a special pin
header with shorter shoulders. But that turned out ok. Yiu do need to
flush-cut all the pins on top and then ideally touch each one again to
melt the shap cut to a smooth dome. But that's easy. The new board is no
different. The Qwiic connector is new, but it is 2.95mm, and so that's
that, no worry even though I haven't actually got it yet.
So nothing much changes either electrically or physically, at least not
in terms of wondering if it will work or not.
In the mean time, while waiting for the new boards I have had the
original board installed in a 100 with the memory power switch turned
off, and it's still reading 2.69v at the cap right now after... 4 or 5
days so far? My original email was 4 days ago and I had done over 24
hours of testing at that time.
This is good.
The leakage thing is this:
First, background, the REXCPM lives in a socket that was only designed
for a ROM, and so that socket only has minimal bus signals. Just power,
address, data, chip-enable, output-enable. It doesn't even have a
write-enable signal like an sram would have. What matters in this case
is among the things it doesn't have is it has no provision keeping
something powered-but-disabled while the machine is turned off. It just
has a VCC pin the dies when everything else dies.
The system bus has a bunch of other signals, including RAM_RST, which is
held *high* the entire time the machine is "off". This is to keep sram
disabled and asleep by holding their active-low /CE pins high. Not just
for a peripheral connected to the system bus but also all the internal
ram is disabled (or enabled) by that same signal.
I don't know if this signal was originally intended to be used as a
power source vs just a control signal. The internal ram has an actual
VBAT power line on their VCC pins, and there is no pin for that on the
bus socket (even though there are 3 whole un-used pins just going to
waste, and 4 more duplicate GND pins than really needed.)
But a sleeping sram, especially a modern one, draws so little power it's
hardly any different from an ordinary control input. And definitely the
signal was always intended to to be read by many chips at once, and that
creates a load all by itself that is the same as drawing power for some
other use. So REXCPM and QUAD and probably a bunch of other add-ons use
RAM_RST as both power source to keep sram alive, and control signal to
keep sram asleep.
Since the REXCPM doesn't live in the system bus socket, and the option
rom socket has neither VBAT nor RAM_RST pins, it has to get RAM_RST via
jumper wire from the system bus (REXCPM also *drives* RAM_RST when
active, to disable the internal ram, to take it's place, so it needs the
connection for other reasons besides power, but for now we're only
talking about when the machine is asleep and REXCPM is getting power
from RAM_RST). So think of RAM_RST as "VCC" power supply to the REXCPM
when the machine is asleep. It's not a leak or backfeed, this it what
that line is supposed to be doing.
That's the positive power wire, but there must be a matching negative
wire. REXCPM does not need a matching ground wire back to the system bus
to go along with the "VCC" one (RAM_RST) because the option rom socket
DOES have a GND pin, and it's connected to the same GND as the ones in
the bus socket and everywhere else in the whole 100.
The leakage happens when you remove the boards from the 100.
Normally, for any randome device like NODE DATAPAC, or QUAD, RAM_RST
power is coming from the memory battery inside the 100, to the RAM_RST
pin in the bus socket, through whatever device is connected to the bus,
back to the GND pin in the bus socket, and back to the memory battery in
the 100.
In the case of REXCPM, in normal usage, it's slightly detoured but still
basically the same. mem batt pos -> sys bus socket RAM_RST -> REXCPM ->
optrom socket GND -> mem batt neg.
With the boards installed in the 100, but the memory power switch turned
off, and our own battery on the bus board, power is coming from the
battery on the bus board, over the RAM_RST wire to the REXCPM, to the
GND pin in the option rom socket, through the GND plane in the 100, to
the GND pin in the system bus socket, back to the battery on the bus
board. Still no weird leakage or backfeeding in this case. Both the
positive and gnd connections are explicit and normal. The GND is a
little indirect, since it relies on the two sockets being connected in
the 100, while the power wire is a direct point to point connection, but
still, it's just a shared GND, nothing actually weird. Every chip in the
computer actually works the same way, it's just like the RAM_RST wire is
like a bodge wire instead of being a trace in the motherboard.
With the boards removed from the 100, it *should* be dead. There is no
dedicated GND wire to pair with the RAM_RST wire, so, there should be no
power going anywhere. A voltage reading between gnd and any other part
of the REXCPM should just all be 0v, like reading 2 points along a dead
end wire.
But somehow there is power differential between gnd and vmem (or
whatever the positive side of the big cap is, I say vmem because
obviously it powers the sram) on the rexcpm, and 0.7uA of current
flowing along RAM_RST.
So somehow something is connecting from RAM_RST to GND, though, not a
short. And also, connecting from RAM_RST or GND or both to RAM, since
it's the RAM wire that causes it to work, though I'm getting ahead of
myself about RAM.
There are several components that have pins on RAM_RST and on GND, and
one or more of those is leaking or backfeeding. The pins are really just
inputs or outputs that happen to be on the different nets to detect
them, be controlled by them, or exert control on them.
There are only two possible other wires that could be providing the gnd
path, and simple testing by disconnecting/reconnecting things shows that
it's the RAM line. Nothing happens when you connect or disconnect the
/WR line. Which is no mystery if you look at the schematic. The /WR line
doesn't connect to anything on the bus board. It just goes from the pin
on the bus socket to the pin going to the REXCPM. Obviously it's used
for something on the REXCPM, but there is no connection to anything *on
the bus board*. It's can't possibly be part of the cricuit that starts
at the CR1025 and ends back at the CR1025, because it doesn't touch
withe GND or BATT+ or anything else eve indirectly on the bus board.
One for-sure part of the circuit is the NOR chip on the bus board. The
RAM line goes to a gate output pin on that chip, and nothing else.
That chip has no other inputs or outputs on either RAM_RST or GND. That
chip's VCC is not powered by RAM_RST but directly by VBUS, and there is
a diode preventing RAM_RST or BATT+ from going into VBUS. But it does
have it's GND pin connected to the same GND as the battery.
So power is going from BATT+ through a diode and out on RAM_RST,
something unknown happens in the REXCPM, and comes back on RAM, leaks
over to GND inside the 74xx2G02, and GND on the bus board is also BATT-.
(The 2G02 is just a 2-gate version of a quad-gate of the same type on
the original board. The original board has a quad-NOR but only 2 of the
gates are used. So this is a smaller part that only has 2 gates.)
The 2G02 is part of the leakage circuit, but only part. Something
similar also has to be happening in the REXCPM for it to get from
RAM_RST over to RAM in the first place in order for it to arrive on RAM
back at the bust board.
It's a weak resistive and/or parasited / voltage-divided connection
because it's essentially going through a string of diodes along the way
going through all those transistors passively, and maybe also being
voltage divided from having multiple paths where some paths drain some
current to gnd without it having to go through the sram, but a sleeping
sram needs so little that it's still enough. At least while the battery
holds out.
My initial pessimistic reaction from the initial voltage drop might just
because of the normal way batteries work and maybe it's not actually as
bad as I thought, even without a proper gnd connection.
batteries DO have a significant early voltage drop, but then they hang
at an 80% level almost flat for most of their life, and then really die
all at once at the end. I saw a 0.2v drop in one day, and only 0.2 to
0.4 available left before hitting 2.0v, and said "this thing will be
dead in one more day!"
But maybe it would actually just hang there at 2.4v and take all year to
to get down to 2.0v.
And installed in the machine so far seems to be totally fine. Only the
expected amount of voltage drop from the battery to the REXCPM, and
apparently no or not much parasitic loss through the various other
signals connected to the 100 when the boards are installed. I'm still
right at 2.7 or 2.69v right now. That's at the big cap on the rexcpm, so
that's after going through a diode on the bus board and maybe another
diode on the rexcpm, and with batteries that are now only making 2.95v
at the battery.
2.83 at RAM_RST (so 0.12 drop across the diode on the bus board), 2.68
on the positive terminal of the big cap on the rexcpm, unknown at the
actual SRAM on the rexcpm.
If I were to turn the memory power switch on, then there is no battery
drain on the bus board batteries as long as the 100's battery is above 3v.
It's starting to look like even the initial version with no dedicated
gnd connection is worth using after all. It will take months to really
tell though. The only thing I can test faster that would be good to find
out is, if the original simpler version is good for more than a few days
outside of the 100?
It might be that even though there is a large voltage drop through the
crap gnd connection, going from 3v to 2.5v before even starting, and
even though the power available at the REXCPM is under 2.4v after only a
couple days, it might be that it then just hangs there at 2.3v or 2.2v
for months before actually going that last little bit to get under 2.0v.
Even if the gnd connection would be better, that would actually already
be a huge help, and would be simpler for most people to use because you
don't have to modify the rexcpm to bodge on some other kind of
connector. Just plug your existing cable exactly the same as the
original board. And, it's not even a compromise except when removing the
rex from the machine entirely. If you're installing *& removing
intermittently so it's only removed occasionally, then no problem.
months or weeks or even mere days is still way more useful than 15
minutes. If you're just leaving the rexcpm installed all the time and
just want to cover when you leave it in a drawer for a year, looks like
that will be no problem either, because there is a good gnd connection
while installed, and (apparently, seemingly, so far) no parasitical loss.
But I would really rather it *also* work fully well when uninstalled, so
that's why I'm bothering to add the gnd wire somehow.
The connector on the new board is a standard one that is available these
days cheap and pre-made in a few different lengths by searching for
"qwiic" cables. I don't yet know if the 2" or 50mm size is too short or
perfect, but the next size is 4" or 100mm and that should be longer then
needed but not too bad. For model 102 or 200 you want 12" or 300mm.
That's a little longer than needed but the next size down might be too
short. I'm not actually sure yet because the way the new board for
102/200 positions the connector, that may actually save 3 or more inches.
It's easy enough to solder the connector on the new board, easy to
source the pre-made cables (it would be a bear to manually crimp them),
and it's an upgrade in usability in the sense that the connector is
polarized. But one big problem is how to connect to the REXCPM.
There are a couple different options.
The simplest from the user point of view is you can buy pre-made qwiic
cables that have female dupont pins on one end. google "qwiic dupont female"
https://www.sparkfun.com/products/17261
The same or equivalent thing is also available from adafruit, and resold
by at least digikey and mouser, and they have their own stock even when
adafruit is out. I actually got one off the shelf retail at a local
micro-center a year or so ago.
https://www.amazon.com/dp/B09WLRBKWT
This will get you the exact same 3-wire connection as before. No GND
connection. No better but no worse. Though, the wire is there, you could
figure out any old hacky way to connect it you want. Like just solder a
single long stiff bent wire like a paper clip maybe to the big cap and
have it extend far enough forward to clear the other dupont pins and
just stick the female gnd wire on it.
Another idea is desolder the original pins and bodge-wire the new
connector to the original pin solder points and gnd on the back side of
the big cap, and glue it to the pcb to immobilize both the connector and
the bodge wires. Or just leave the connector end loose on flexible wires
and only glue down the solder end, and maybe put some glue and
heat-shrink on the connector end. I'm not sure how easy this will be
until I try it. The connector is a almsot 3mm tall and has to lay on top
of some existing components on the rexcpm, so, there might be a little
problem with height there interfering with the compartment cover. I also
expect almost no glue actually sticks to the connector body, so it might
need mounting tape or who knows what.
Maybe I just change the design again to use some other more convenient
type of connector that isn't quite so small but still small enough?
There are cheap male & female pigtail kits for JST-PH 2.0mm that have a
male connactor crimped on to wires. That would be dead easy to solder
the male side onto the rexcpm.
https://www.amazon.com/dp/B081CRLN8B
And I guess the other end could just be wires soldered to the bus board
too, so that one kit does everything, but I actually really prefer the
original arrangement with only fixed connectors (be they male or female)
on the boards and wires only in the cable between. I would rather figure
out some way no matter what it takes, to have just fixed connectors on
the boards.
Maybe I can desolder the original pins but only to slide them up
slightly so that they are as high in the air as they can go before
hitting the compartment cover, and maybe that leaves enough space
between the pins and the pcb to solder the connector to the underside of
the pins, and the pins provide the rigid mounting. Then the gnd wire can
be a loose bodge wire glued down. Maybe I can add a 4th rigid pin
extracted from some other generic header or connector, from the rear of
the cap over to the connector, to solder to one of the mechanical
mounting solder pads on the side of the connector too, as well as the
gnd pin.
The rexcpm side of things is now the unknown tricky part but hopefully I
figure out some plan that isn't too terribly hacky and is actually
doable and solid.
--
bkw
On 9/14/23 14:53, Ken St. Cyr wrote:
Great work! To make sure I’m following - the REXCPM relies on having a
ground path through the socket, so the idea is to swap out the 3-wire
JST connection for a 4-wire version, which adds the ground pin... And
you’re saying the only reason it works outside the unit right now
without the ground path is because there’s leakage on the RAM_RST line?
I'd like to give it a shot ... have you done a prototype run of the 4x
battery board yet?
//Ken S
------------------------------------------------------------------------
*From:* M100 <m100-boun...@lists.bitchin100.com> on behalf of Brian K.
White <b.kenyo...@gmail.com>
*Sent:* Wednesday, September 13, 2023 3:13:53 AM
*To:* m100@lists.bitchin100.com <m100@lists.bitchin100.com>
*Subject:* Re: [M100] rexcpm battery
Let's give THIS a try...
https://photos.app.goo.gl/JaSofoK2VotHzVco9
<https://photos.app.goo.gl/JaSofoK2VotHzVco9>
The connector is a standard JST-SH, and cables are available pre-made in
both 2" for the 100 and 12" for the 102 or 200.
There's even an off-the-shelf cable that has dupont sockets on one end,
so you could connect to an unmodified rexcpm.
So now we have
120 mah
gnd connection
polarized plug
I think we're looking at at least 2 years of shelf life now.
--
bkw
On 9/12/23 15:05, Brian K. White wrote:
I've done a little more testing.
The GND path is backfeeding only via the RAM line. Nothing on /WR.
(the only possible connections)
I am measuring only 6.6uA on the RAM_RST line between the two boards
with the boards outside of the 100.
That is a little high but actually not too bad. I would hope for under
5uA ideally but 7uA is in the same class.
At that rate, nominally the batteries should last over 11 months. And
at that low current there shouldn't be much voltage drop across the
diode on RAM_RST, and indeed there isn't. I only lose about 0.1v from
the battery to RAM_RST.
The bulk of the voltage drop is happening along the GND path, and
things change a lot when the boards are installed in the 100 and the
100 provides a real GND connection between the two boards.
With both boards outside of the 100, I currently measure
(the batteries are draining as time goes on so these numbers all
change a little each time I check something, so I'll keep saying the
battery voltage along with whatever else I'm talking about)
2.99v between BATT- and BATT+
2.91v between BATT- and RAM_RST (so practically no drop through the
diode on the bus board)
2.76v between BATT- and CAP+ (it's not labelled CAP+, I just mean the
positive side of the big cap) on the REXCPM (after giving it time to
drain away the 5v from the 100)
2.26v between CAP- and CAP+
So there is not actually much drop from BATT+ along RAM_RST to REXCPM
VCC, but a 0.5v loss along the GND-via-RAM path.
Next, when I provide an actual GND connection between the boards by
plugging them in to the 100 with the memory power switch turned off,
the current draw actually increases slightly to 7.2uA (after an
initial spike probably charging the cap), but the voltage drop
decreases. The battery is currently at 2.99v and I get 2.72v at the
cap instead of 2.26v
While writing this the current along RAM_RST very gradually dropped
further to 6.9uA. Maybe it will continue creeping down to settle at
the same 6.6-6.7 eventually?
This suggests that maybe the batteries would provide a worthwhile
shelf-life extension after all, at least while installed in the
machine which provides a real GND connection.
60mAh at 8uA works out to just over 10 months. (according to
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-battery-life) <https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-battery-life)>
That's worth doing I think. Especially when added to the 100's own
shelf-life, that should work out to maybe 1.5 years total.
I still have to check the actual current drain at the battery. My
current measurements above were just on the RAM_RST line from the bus
board to the rexcpm, not at the battery itself. It's possible the
battery is seeing more total current than just that through additional
paths like reverse leakage through the schottkey from BATT+ to VBUS,
and through RAM_RST into the 100.
Anyway it looks a lot more promising than I thought at first.
As for getting that same long shelf-life outside of the 100, one idea
might be to mod the REXCPM to use a 4-pin JST-SH connector and use the
same connector on the bus board, and a pre-made cable assembly.
Those are very small connectors and would be difficult to crimp the
female connectors on the cable by hand, but they sell those pre-made
in suitable lengths. And the male connectors are only 2.95mm tall and
7mm wide (really only 6mm but the flanges on the cable connector are
7mm) and that fits easily on both the rexcpm and the bus board.
On the rexcpm side it would have to be bodge wired to the 3 existing
pins and the 4th pin to the negative side of the big cap, then secured
with glue or mounting foam tape.
On the bus board it's nothing. The connector footprint fits right
where the existing 3-pin header is now.
https://mou.sr/3K9FwRe <https://mou.sr/3K9FwRe>
https://mou.sr/44Pcako <https://mou.sr/44Pcako>
or
https://www.digikey.com/short/7hb4cm4q
<https://www.digikey.com/short/7hb4cm4q>
https://www.digikey.com/short/wzj8ppjb
<https://www.digikey.com/short/wzj8ppjb>
And as an added bonus, the new cable is fully polarity keyed on both
ends too.
--
bkw
--
bkw
