Solution: N900 Charging from dumb power sources *UPDATED* - Page 6 - maemo.org

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jeffsf	2010-01-01 , 16:44
Posts: 44 \| Thanked: 24 times \| Joined on Dec 2009 @ San Francisco	#51

It is likely firmware somewhere, but not clear if it is flashable firmware.

There is most likely a charging circuit in the device, either discrete or perhaps part of the USB chipset, that handles the negotiation of draw from the USB leads. Since the N900 will charge without being booted up, the logic of charging can't be completely handled by Maemo. However, it is possible that the configuration of the charging circuit can be done through the appropriate bus from Maemo to change the behavior of the charging circuit to one that is more reliable.

Edit -- Apparently there has been at least one bug related to failed detection of USB and lack of charging addressed in
https://bugs.maemo.org/show_bug.cgi?id=6004

This has been fixed in package
kernel 2.6.28-20094603+0m5
which is part of the internal build version
2009.46-16

This appears prior to the rumored candidate firmware "PR-11" based on week 51 builds.

Last edited by jeffsf; 2010-01-01 at 16:55.

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Rushmore	2010-01-01 , 18:26
Posts: 1,255 \| Thanked: 393 times \| Joined on Oct 2009 @ US	#52

Originally Posted by jeffsf

It is likely firmware somewhere, but not clear if it is flashable firmware.

There is most likely a charging circuit in the device, either discrete or perhaps part of the USB chipset, that handles the negotiation of draw from the USB leads. Since the N900 will charge without being booted up, the logic of charging can't be completely handled by Maemo. However, it is possible that the configuration of the charging circuit can be done through the appropriate bus from Maemo to change the behavior of the charging circuit to one that is more reliable.

Edit -- Apparently there has been at least one bug related to failed detection of USB and lack of charging addressed in
https://bugs.maemo.org/show_bug.cgi?id=6004
This has been fixed in package
kernel 2.6.28-20094603+0m5
which is part of the internal build version
2009.46-16

This appears prior to the rumored candidate firmware "PR-11" based on week 51 builds.

That was my point last month, that there is some form of handshake taking place and is likely by design to avoid some of the more "cheap" charges from frying the device. Very unlikely this could happen, though the design is more complex than simply allowing all dumb chargers based on correct current draw to work. This is why I suggest it was designed this way on purpose.

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woodyear99	2010-01-01 , 20:09
Posts: 318 \| Thanked: 49 times \| Joined on Nov 2009	#53

Originally Posted by jeffsf

It is likely firmware somewhere, but not clear if it is flashable firmware.

There is most likely a charging circuit in the device, either discrete or perhaps part of the USB chipset, that handles the negotiation of draw from the USB leads. Since the N900 will charge without being booted up, the logic of charging can't be completely handled by Maemo. However, it is possible that the configuration of the charging circuit can be done through the appropriate bus from Maemo to change the behavior of the charging circuit to one that is more reliable.

Edit -- Apparently there has been at least one bug related to failed detection of USB and lack of charging addressed in
https://bugs.maemo.org/show_bug.cgi?id=6004
This has been fixed in package
kernel 2.6.28-20094603+0m5
which is part of the internal build version
2009.46-16

This appears prior to the rumored candidate firmware "PR-11" based on week 51 builds.

That particular bug is annoying it happens all the time once my phone is locked, have to unlock and then plug in the phone each time I charge via usb.....

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jeffsf	2010-01-01 , 20:33
Posts: 44 \| Thanked: 24 times \| Joined on Dec 2009 @ San Francisco	#54

Originally Posted by Rushmore

That was my point last month, that there is some form of handshake taking place and is likely by design to avoid some of the more "cheap" charges from frying the device. Very unlikely this could happen, though the design is more complex than simply allowing all dumb chargers based on correct current draw to work. This is why I suggest it was designed this way on purpose.

As an EE, I find that hard to swallow.

As long as, within reason, the proper voltage is being supplied to the USB port, then it is up to the charging circuit to determine how much current it will draw. If you hook a 5V supply to a 25 ohm load, you'll consume 1 Watt from any supply can source 200 mA or more.

There is some complexity with internal resistance of the supply, but that is a secondary effect which, if anything, a "smart" charger could be aware of, dropping its current demands when the supply voltage starts to droop, hence automatically accommodating everything from the poorly-regulated CLA from the guy with the port-a-table on the corner, through my 350 mA Plantronics charger, to a Nokia OEM charger can can source 1200 mA.

That the N900 allows "shorted data" USB supplies to bypass the handshake contraindicates the hypothesis that this is somehow supposed to "protect the charger."

I find it reasonable that Nokia is worried about the case where

I hooked my N900 to my laptop and it fried my $3000 laptop.

but find it unreasonable that they are worried enough to "lock out" virtually all non-active-USB-hub devices (including problems with their own DC and AC adapters) for the case where

I hooked my N900 to my no-name CLA and it fried the CLA.

which generally would be prevented by any well-designed "smart charger" circuit (which would be looking at the charging current and voltage both),
or even

I hooked my N900 to my no name CLA and it cooked my N900.

"Well, what did you expect?" would be the polite form of the comments someone would likely get in the latter two cases. In neither case, if the charger had the data pins shorted, would the apparent N900 behavior, have saved the charger or the N900.

__________________
Only Nokia since 1995 -- But I've lost the faith in the last year.

Alas, why I'll likely never recommend the N900 to my friends

Non-nerd apps; USB charging - generic and reliable; Distinctive ring tones; GPS failure to lock; Competitive "maps" application; Supported MMS, iSync, Bluetooth

(The feeble FM transmitter bugs me too)

Last edited by jeffsf; 2010-01-01 at 20:49. Reason: Clarity

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TA-t3	2010-01-01 , 20:42
Posts: 3,841 \| Thanked: 1,079 times \| Joined on Nov 2006	#55

As another EE I can only agree.. the charger must supply the proper voltage, and it should be clean. If those are OK there's no way the charger could fry the N900.

__________________
N800/OS2007|N900/Maemo5
-- Metalayer-crawler delenda est.
-- Current state: Fed up with everything MeeGo.

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Rushmore	2010-01-01 , 20:42
Posts: 1,255 \| Thanked: 393 times \| Joined on Oct 2009 @ US	#56

Originally Posted by jeffsf

As an EE, I find that hard to swallow.

As long as, within reason, the proper voltage is being supplied to the USB port, then it is up to the charging circuit to determine how much current it will draw. If you hook a 5V supply to a 25 ohm load, you'll consume 1 Watt from any supply can source 200 mA or more.

There is some complexity with internal resistance of the supply, but that is a secondary effect which, if anything, a "smart" charger could be aware of, dropping its current demands when the supply voltage starts to droop, hence automatically accommodating everything from the poorly-regulated CLA from the guy with the port-a-table on the corner, through my 350 mA Plantronics charger, to a Nokia OEM charger can can source 1200 mA.

That the N900 allows "shorted data" USB supplies to bypass the handshake contraindicates the hypothesis that this is somehow supposed to "protect the charger."

I find it reasonable that Nokia is worried about the case where

I hooked my N900 to my laptop and it fried my $3000 laptop.
but find it unreasonable that they are worried enough to "lock out" virtually all non-active-USB-hub devices (including problems with their own DC and AC adapters) for the case where

I hooked my N900 to my no-name CLA and it fried the CLA.
or even

I hooked my N900 to my no name CLA and it cooked my N900.
"Well, what did you expect?" would be the polite form of the comments someone would likely get in the latter two cases.

My undergrad is in computer electronics, so appreciate your points, but I am trying to give Nokia the benefit of the doubt for a seemingly odd design decision

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jeffsf	2010-01-01 , 20:54
Posts: 44 \| Thanked: 24 times \| Joined on Dec 2009 @ San Francisco	#57

Originally Posted by Rushmore

I am trying to give Nokia the benefit of the doubt for a seemingly odd design decision

We're certainly dealing with a few "interesting" implementation artifacts, and I expected that when I bought a 1st-generation device.

Maybe it is that if you can't charge the device, you don't see the rest of them?

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denix	2010-01-04 , 01:47
Posts: 33 \| Thanked: 8 times \| Joined on Dec 2009	#58

Originally Posted by jeffsf

We're certainly dealing with a few "interesting" implementation artifacts, and I expected that when I bought a 1st-generation device.

Isn't it a 4-th generation device?

What a shame - all my backup batteries are useless now! And they always worked with the rest of my equipment - dumb/smart phones, headsets etc...

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shadowjk	2010-01-04 , 08:29
Posts: 1,258 \| Thanked: 672 times \| Joined on Mar 2009	#59

Originally Posted by jeffsf

As an EE, I find that hard to swallow.

As long as, within reason, the proper voltage is being supplied to the USB port, then it is up to the charging circuit to determine how much current it will draw. If you hook a 5V supply to a 25 ohm load, you'll consume 1 Watt from any supply can source 200 mA or more.

The USB standard allows devices to draw 100mA. A full 500mA can be provided if the device negotiates this with the connected host. The host's usb ports should be overcurrent protected and shut down the power if the device consumes too much power.

This is why you can't just use the logical method of draw power until voltage drops.

A new charging profile was added to the usb standard, which includes signaling for letting the device know when it can behave as if it was connected to a charger. For cheap/dumb chargers, the simple signaling of d+ and d- shorted together is used.

The N900 complies with the USB charging standard, but the problems come from everyone using random nonstandardized charging schemes before the USB charging standard was finalized.

It's worth noting that there's no usb charging over miniusb, for example..
[/quote]

That the N900 allows "shorted data" USB supplies to bypass the handshake contraindicates the hypothesis that this is somehow supposed to "protect the charger."

The usb charging supplement is available to the public. Might contain some clues to the design decisions made

I find it reasonable that Nokia is worried about the case where

I hooked my N900 to my laptop and it fried my $3000 laptop.

iirc there already was a case where someone hooked it up to a freerunner and nearly fried it. A prototype n900 I think. Sadly, in practice it seems many computers lack effective overcurrent protection on USB and will happily shove 20 amps over it until something fries and voltage drops from 5 to 0 in one go

In neither case, if the charger had the data pins shorted, would the apparent N900 behavior, have saved the charger or the N900.

I think the datapins shorted together decision might have been to make the most unambigous way of distinguishing between a computer's usb power and a charger, some computers give power on usb when off/suspended, but have nothing on the datapins.

To me it looks like they (the usb standards body) picked the most reliable and inexpensiv way of distinguishing between a real usb host and a charger.

Last edited by shadowjk; 2010-01-04 at 08:43. Reason: qupte/snip fail