On 9/22/23 18:09, Brian White wrote:
> 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
>
That was sent from my phone just to get the numbers down.
What I GET from it is,
> While not installed in 100:
> BATT 2.99v
> RAM_RST 2.91v 7uA
> SRAM VCC 2.76v
* Draws 7uA while being powered from the batteries alone.
* Almost no voltage drop from the battery to RAM_RST.
* Still very low voltage drop all the way to the actual vcc pin on the
sram (vs just at the cap), so a simple battery life estimate based on
just nominal mAh capacity and uA drain is correct with no unaccounted
further drops.
> While installed in 100, power on, wall power, original tandy 6v adapter:
> RAM_RST 4.85v
> BATT+, batts installed 3.0v
> BATT+, batts not installed 3.65v 0.5uA
* 0.5uA reverse leakage through the BAT54C
* trying to pull the cells up not only over their nominal 3.0v but even
over their actual measured brand new initial 3.2v.
And the datasheet for a CR1025 cell says "Max Rev Charg: 1 microampere"
So, in nice cool under 25c conditions there isn't a problem, but that
0.5uA goes up over 1uA in temperatures that are still possible human
environment ambient.
The risk is only while powered on, not while say, sitting in a hot car
not being used.
Then again, unattended running 24/7 has always been a use case for these
machines, like famously the EME Online Weather Logger setup. So it could
actually be on and running in a pretty hot environment.
So I guess stick with the plan to call for BAV170 or other low reverse
leakage diode.
Another thing I looked at, which also in the end resulted in no change:
I read that tantalum caps have leakage current and maybe not ideal for
long term battery standby applications.
So I removed the tantalum cap and still measured 7.4uA when powered from
the battery.
So the drain is not from tantalum leakage current.
I was hoping for a possible hack to replace the tantalum with a ceramic
to trade away some minutes of battery-change time for some more years of
battery standby time.
But if it's still drawing over 7uA with no cap installed at all, then
it's not going to get any better.
I can't believe the sram chip is drawing 7uA itself.
My REXCPM originally came with a CY62177EV30 whose datasheet says has a
standby current of 3uA typical, max 25uA, so IT might possibly draw
7-8uA, but I replaced that with a RMLV3216AGSA to upgrade it from 2M to
4M, and that datasheet says the typical standby current is only 0.6uA,
though when you look at the tables vs the summary in both sheets, they
actually look like pretty similar min max range. They may both actually
only draw about 1uA normally or up to 24-25 in worst case high temp.
There is at least a 3.3v voltage regulator and who knows what else
besides just the sram being powered while on standby.
I thought maybe the 4 batteries was a little unnecessary overkill but I
guess it's worth it after all, since even with all 4, the estimated life
is still only about 2 years.
It would have been nice to go back to 2 cells and use the original big
standard header pins (4 big pins still fits where the 3 pins were on the
first version). Actually no reason I can't still just make an option for
that.
So I think this is pretty much it, as good as it's gonna get without
picking apart the rexcpm itself to identify some possible improvement to
the standby mode, like cutting out the voltage regulator and switching
over to a direct connection on standby, or maybe just replacing the
regulator with some other model, maybe finding pullups/downs that could
be replaced with higher value resistors, etc.
But 2 years is good enough to be worth doing I think, and the default
hookup method without hacking a qwiic connector onto the rexcpm is easy
enough.
bkw
