USB Bus power >100mA

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Hi Everyone,

I started a thread earlier about USB bus powering an instrument that needs
to draw more than 100mA and less than 500mA. The USB controller is part of
a processor and not really negotiable. It is a high speed device.

I think this problem is quite common but I have not found a great solution.
So I am putting it out here for ideas.

Here are the constraints:

1. The device will draw more than 100mA before it can request 500mA.
2. Even if power could be constrained to 100mA, there is no direct way to
support suspend current.

I have also looked at a variety of USB Power Managers but I don't see how
these get around the problem.

Several ideas have been considered:

Idea 1:

Use a two port bus powered hub. Connect a low power USB device like a PIC
or FTDI, etc that requests 500mA. This device just controls a switch that
supplies almost 500mA to the second device. The second device looks to the
USB bus like a self powered device.

This takes a 2 port bus powered high speed hub and another microcontroller
or USB peripheral. Lots of parts to solve a basic problem.

Idea 2:

Just use a bus powered 2 port hub. Since it is a bus powered hub, it
presumably already has permission to draw 500mA. Connect the target device
to this internal hub. It gets 500mA less the hub current.

I assume that this probably breaks some USB rules, but at least eliminates
the extra USB device from Idea 1.

Idea 3:

This is where you tell the rest of us, the clever way to solve this
problem......  



If either Idea 1 or Idea 2 are practical, what parts make the most sense to
implement the circuits. The idea is to keep the wasted power to a minumum
and the size and cost down.


Thanks


Al Clark
Danville Signal









Re: USB Bus power >100mA
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Ideas 1 and 2 look practical on the surface, if perhaps a bit "cheaty".

In your previous threads you mentioned that the devices in question
would be a an FPGA or a high-power processor.  Is it possible to get
either of them shoe-horned to less than standby current by running them
really slowly, then ramp up the clocks when you get the OK?

Alternately, are you sure there aren't any USB PHY devices that can
handle the bus configuration and standby queries, that maybe has an
EEPROM that lets you configure it for how much current to ask for?  I
checked the FTDI website and I'm mildly surprised to see that they don't
address this market already -- but they do have chips that are good for
12Mbaud throughput, which is certainly a considerable amount of data.

--

Tim Wescott
Wescott Design Services
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Re: USB Bus power >100mA

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Maybe, I am all for elegant.





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Is it possible to get
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The DSP power is dominated by leakage current. This is really more a
function of temperature than MIPs or Clock.

I probably can't make it even without the DSP




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You would think so?

I haven't found one yet. I would love for someone to point me to the part I
missed. I have looked.


Al

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Re: USB Bus power >100mA

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As soon as you get your board done, someone will come out with what you
need :).

What about a micro with built-in USB?  Maybe you could get lucky with
some combination that'd let you pump data out to your 'real' processor.

--
Tim Wescott
Control system and signal processing consulting
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Re: USB Bus power >100mA
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You have full control of the software, right?

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This is a violation of the in-rush current requirements.
But why? Start your system in 2 stages: Before and after set configuration.

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Why not?
If the USB controller is built into the processor, the designers most likely
considered this problem.
(suspend current is 2.5 mA)

What kind of host system will your device be connected to? PCs?
Is USB certification required?

A way to bypass the suspend current problem is to add a (small/useless)
battery, since battery powered devices are allowed to draw 100 mA during
suspend.

Leo HavmÝller.


Re: USB Bus power >100mA

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Actually USB is not specifically part of the processor. It uses a ULPI
transceiver.





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I don't need official certication, but it must work reliably. Assume the
host is a modern noetbook PC running Windows 7 or XP.





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I haven't heard this idea. I have been wondering if there is a way to take
advantage of charging circuits in some way.


Thanks

Al




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Re: USB Bus power >100mA
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Wait a minute -- aren't there Blackfin devices that have embedded USB?

For that matter, I _know_ that TI's Stellarus line has some ARM devices
with USB.  I don't know if they'd have the throughput to act as a
communications front end for you, but I _do_ know that their SSI ports
have at least minimal DMA functionality -- there may be enough stuff
there to put together something.

--

Tim Wescott
Wescott Design Services
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Some Blackfins do have USB support. We make a Blackfin 527 module that has
this.

I have a specific self powered design that already exists. I am not trying to
find a new processor that has USB integration.

What I am really trying to do is solve a fairly common problem where USB bus
power is desired and it is difficult to meet both suspend current and 100mA
requirements.

This is a common problem when using ULPI transceivers since the "brains" are
often external processors that are not specifically designed to manage low
power. DSPs and FPGAs are typical examples of this. The problem is generally
more severe for high speed requirements because high speed implies devices
that can take advantage of the data.

Al

Re: USB Bus power >100mA
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Which is precisely why I was suggesting (not very directly, I admit)
that you investigate the possibility of finding the smallest, cheapest
processor that will work as a USB communications peripheral.  It's not
the most elegant solution -- that would be a true high-speed interface
chip along FTDI lines but capable of 400MBPS.  But it may get you where
you want to go, and be cheap and functional enough to serve.

In fact -- how about I figure out how to make it work with a Stelarrus
chip.  I'll program it, grind the part numbers off of it, put a Wescott
Design Services sticker on it, and I'll sell it to you for about twice
the DigiKey 1ea pricing for the blank chip.  Then you won't have to
think about a whole ARM core that's sitting there doing almost nothing,
while pretending to be about 500 gates.

--

Tim Wescott
Wescott Design Services
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Re: USB Bus power >100mA
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Well, then I think you can solve the problems with some simple power
management in software.
Start your system in multiple stages. Dont power up peripherals and clock up
your CPU until you receive set configuration.
You will want to reduce current sonsumption during suspend as much as
possible. Consider the case when your device is connected to a laptop that
is sleeping; Your device should not draing the laptop battery. Shutdown
everything, clock your CPU way down (100kHz or less), and make a tight loop
that polls your USB controller for resume/reset events and nothing else.

Leo HavmÝller.


Re: USB Bus power >100mA

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I have a somewhat similar situation but I'm using the USB port for power
only.  I am not using USB for any communications so I'm not enumerating
a device.  When I first started worrying about this I was thinking about
using a FTDI chip and program it for 500mA power level and then figure
out some way to manage a load switch with it.  However, what I have
found is that the usb ports will give me full power anyway.

I got a 10 ohm, 5W resistor and started hanging it across USB ports and
measuring the voltage and current.  I was getting 4.8/4.9 and close to
the 500mA. In the 6 I tried, Dell, HP and generics, none of them
did a 100mA limit.

Again - please note that I am not using the USB protocol and am not
enumerating any device.  I'm just drawing power and does not seem to be
an issue.  I'm also not having to deal with a generic USB implementation.
I'm lucky this time in that I have a very defined set of host that I have
to deal with.  It sounds like you have an integrated system so if you
have control over the host look at any software power management settings.
Linux can be told to never suspend a device.

You might use a PIC, AVR, (pick your uC) as a USB "gateway" for your
heavy power device.  Have it enumerate for 500, turn on a load switch
and start relaying traffic.  USB on one side and some serial or
parallel interface on the heavy power side.
 

--
Joe Chisolm

Re: USB Bus power >100mA
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The ports are not limited. It's just that the OS keeps track of used
current. If a device requests, say, 300 mA from a bus powered hub and some
other device already got 300 mA by request, the second device's request will
not be granted.

Meindert



Re: USB Bus power >100mA
GdnafNjsg1VJfTnZ2dnUVZ snipped-for-privacy@earthlink.com:

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I have done this experiment also. The only place it seems to matter is the
place where bus power is actually important - notebook PCs.

These computers actually care about power consumption and may exert more
control.


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settings.

My application can not assume that I have control of the PC side. The OS is
likely some flavor of Windows.



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This is really just a subset of "Idea 1". I need the high speed USB
interface. It's not negotiable.

Thanks

Al




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Re: USB Bus power >100mA




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1. It will just work in the 99% of cases.

2. However, if you are constrained to one USB socket, the only way to
satisfy the above without breaking some rules is charging a battery or a
supercap from USB and using them at the moments when you need some extra
power.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com


Re: USB Bus power >100mA

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Thanks Vlad

Al

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