Ignore Suspend Current in USB Bus Pwr application?

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I am looking at an device with a MCU managing a USB high speed peripheral.
There is a SMSC USB3318 acting as a ULPI transceiver.

It is desirable to be able to bus power the device but it may be very
difficult to implement suspend mode on the USB. Changing the MCU is not an
option in this case.

I can manage the current to less than 100mA before needing 500mA (I could
probably use more).

As I understand the spec, suspend mode is required for USB compliance. What
if we can live without full compliance?

Suggestions?  


Al Clark

 

Re: Ignore Suspend Current in USB Bus Pwr application?
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It certainly won't pass if you draw more than 100mA before it has been
configured.

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You can, I believe, generally get a dispensation for the suspend mode
current - the normal limit is 500uA but 2.5mA is permitted.  If your
device can manage it you can rely on the fact that the current is
averaged over one second (see 7.2.3, USB 2.0).

Whether you need USB certification may depend upon your customers.

The only way that I've been able to get near the suspend requirements in
the past is to shut everything down and have the USB device
(effectively) control the MCU.  In some circumstances this isn't ideal
and self-powered is the way to go.

    Andrew

Re: Ignore Suspend Current in USB Bus Pwr application?

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So if we assume that the current can never be actually suspended (not even
close to a few mA), then the device will still work but it will never be
USB compliant in Bus Power.

The situation I have in mind is an instrument tied to a notebook computer.
It is really inconvenient to have an external supply in this case. The
instrument would probably still have an external power input to allow
optional self power operation and therefore avoid the issue.


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I too have used this method, but it doesn't play well with my current
design constraints.

Thanks

Al


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Re: Ignore Suspend Current in USB Bus Pwr application?
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Yes.  In normal use, I've never had a problem with high suspend
currents.  Clearly overcurrent (>500mA) is a different matter and all
sorts of alarms go off then!

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I've a number of USB instruments that aren't certified (though the
controller chips are) but perform perfectly well.  For my projects, I
normally run through all the USBCV tests anyway (irrespective of whether
I am actually trying for certification).

The suspend current has always seems way too low to me.  I can
understand the reasons for it but it you want any sort of intelligence
in your (slave) device then it is a real pain and requires a lot of
effort to achieve.

    Andrew

Re: Ignore Suspend Current in USB Bus Pwr application?
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Also my experience.

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Me too.


The change from 500uA to 2.5mA did the difference between "completely
impossible" to "should be possible, so must be achieved".
All my recent devices are compliant, but I cannot remember the last time a
client actually wanted to do certification.
But suspend sure does cost a lot of software development time.
Suspend is fine for the few devices where remote wakeup is relevant, but for
all the rest, im really missing a bmAttributes bit to say "suspend is
useless to me, just cut the damm power".

Leo Havm°ller.


Re: Ignore Suspend Current in USB Bus Pwr application?
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Even though the spec only defines 2.5W power (500mA @ 5V) there are USB
hubs that provide 5W.  We have used these in the past.  And have used
non-suspend supporting designs without issue.  Never certified.

Chris Felton

Re: Ignore Suspend Current in USB Bus Pwr application?
email.me:

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not
even

This may not exist and further show my USB ignorance, but is there some
kind of simple hub-like device that could act as a USB power management
device that would be just smart enough act as a pass thru extender and
manage power during enumeration and suspend.

It seems to me that this would be pretty handy for a lot of designs.

Al







Re: Ignore Suspend Current in USB Bus Pwr application?
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Hopefully, I didn't mis-speak.  Doing a quick google I can't find a
similar USB hub that I thought existed.  I will need to go through my
emails and see if I can find the hub that we used in the past.  We used
this for a board that was normally self-powered but when we were testing
the board it was simpler to run off USB power.

Chris

Re: Ignore Suspend Current in USB Bus Pwr application?
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An alternative is to have two USB connectors, as found on some of the
portable hard drives, one of them only provides power.

    Andrew

Re: Ignore Suspend Current in USB Bus Pwr application?
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Here is the USB hub, I believe we used, it has two "high" powered ports.

http://www.targus.com/us/productdetail.aspx?skuAC%H81US .  It is a little
spendy, but if 5W is enough this might work.

Chris

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