So, going with the proposed initial design, I created a report descriptor to
describe the battery. I thought that I'd then see how NUT responded to it.
The good news is that the descriptor showed up and all of the paths were
correct. Unfortunately, after that, nothing happened...no reports were
requested, and it just kept timing out. Is there some minimum amount of
functionality I need to provide in this report descriptor in order for NUT to
work correctly? I assume it's not requesting reports because there is nothing
it wants to know about.
Here is what I have so far:
USAGE_PAGE (Power Device)
USAGE (UPS)
COLLECTION (Application)
USAGE (BatterySystem)
COLLECTION (Physical)
REPORT_ID (1)
USAGE (Voltage)
REPORT_SIZE (8)
REPORT_COUNT (1)
UNIT (SI Lin:Volts)
LOGICAL_MINIMUM (0)
LOGICAL_MAXIMUM (255)
FEATURE (Data,Var,Abs)
REPORT_ID (2)
USAGE (Temperature)
REPORT_SIZE (8)
REPORT_COUNT (1)
LOGICAL_MINIMUM (0)
LOGICAL_MAXIMUM (255)
UNIT (SI Lin:Temperature)
FEATURE (Data,Var,Abs)
USAGE_PAGE (Battery System)
REPORT_ID (3)
USAGE (RemainingCapacity)
REPORT_SIZE (8)
REPORT_COUNT (1)
LOGICAL_MINIMUM (0)
LOGICAL_MAXIMUM (100)
FEATURE (Data,Var,Abs)
END_COLLECTION
END_COLLECTION
From: Nut-upsdev
[mailto:[email protected]] On Behalf
Of Rob Groner
Sent: Wednesday, March 12, 2014 10:48 AM
To: [email protected]
Subject: Re: [Nut-upsdev] Developing the UPS side of the UPS-NUT equation (via
usbhid)
The syntax is beginning (just beginning) to make sense to me now, so I'm
looking now to try and implement just a few items in the report descriptor, and
then I figured I can progress from there if all goes well.
The power HID docs seem to encourage drawing out your system first, and then
letting the report descriptor flow from there. So, my very simplified system
goes like this:
[Battery]===DC Flow ==>[Power Summary]
All of the drawings seem to end in a power summary, and the doc seemed to
stress that it was essential to power management software being able to
effectively use it, so I'll include it.
I think I understand how the usages work now....they're basically like
establishing the namespace (like com.java.class, etc). I had been confused
before when I would see multiple usage() statements all following each other,
but seeing it in the nut output as the path makes it more clear.
So, pretty much everything should be under the UPS usage. Then I would define
things under the battery usage, then the flow usage, and then the power summary
usage...right?
Where I'm still foggy is two things....first collections. I'm pretty sure
there's supposed to be an application collection, and then I usually see a
physical collection, but I'm still not completely understanding what their
purpose is.
The second thing I still don't understand is the minimum/maximum descriptors.
Ok, Logical min/max I get...that's just the range of the actual value in the
report...basically a way to indicate if it is unsigned or signed. If you only
have an 8 bit value specified, then I'd guess you can't have a logical max
greater than 255. But physical min/max are a bit confusing. Is it just a
means of scaling or offsetting the logical value?
So, for instance temperature. Our UPS has a temperature monitor on it, but HID
temperature values are in Kelvin. I could define an 8-bit temperature field,
with a logical min/max of 0/100, and a physical min/max of 273/373. So if I
pass 50 as a value, then it gets read as 323 on the PC side? And if the
physical range is much greater than the logical range, then it just scales it?
The other min/max I don't quite understand is usage min/max. I'm thinking they
have something to do with the usages "OutputID", "FlowID", etc.
Thank you for the help so far. I really wish there were better learning docs
out there for the un-initiated.
Rob
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