[sdiy] USB Question

Wed Sep 23 19:37:23 CEST 2015

I believe I can use the uP VBUS detect pin to detect the B connector. 
Then its just a matter of finding a power switch that can limit current 
on the A connector, and can be turned off of course.

Thanks everyone!

On 9/23/2015 10:23 AM, Vladimir Pantelic wrote:
>
> yes, that is doable
>
> On Sep 23, 2015 7:08 PM, "Tim Ressel" <timr at circuitabbey.com 
> <mailto:timr at circuitabbey.com>> wrote:
>
>     This will be for USB Midi, so the host connector needs to be
>     powered and the device connector does not. But I should be able to
>     sense power on the device connector and switch accordingly? 
>     Default to host and switch to device upon power on the device
>     connector?
>
>     --TimR
>
>     On 9/23/2015 9:56 AM, rsdio at audiobanshee.com
>     <mailto:rsdio at audiobanshee.com> wrote:
>
>         One correction to my comments: Whether your Device mode is
>         self-powered or not, you always need to be able to read the
>         power coming from the USB B connector to ensure that your CPU
>         does not enable or drive the data lines unless power is
>         present. That's one of the basic rules of USB. So, this means
>         that your source for USB power over the A connector cannot
>         connect to the power on the B connector, otherwise you'll
>         never know.
>
>         The only thing missing from the EK-TM4C1294 LaunchPad is an
>         example circuit for a system with both A and B connectors, so
>         that's where we're getting into new territory. If anyone knows
>         of an evaluation board that does have both A and B -
>         especially with schematics freely available - then please
>         share with the list.
>
>         B
>
>
>         On Sep 23, 2015, at 9:47 AM, rsdio at audiobanshee.com
>         <mailto:rsdio at audiobanshee.com> wrote:
>         On Sep 22, 2015, at 11:28 PM, Vladimir Pantelic
>         <vladoman at gmail.com <mailto:vladoman at gmail.com>> wrote:
>
>             On 23.09.2015 08:10, Tim Ressel wrote:
>
>                 I need to design a usb interface that does both host
>                 and device, but not
>                 at the same time. There will be an A and B connector
>                 on the front
>                 panel.  All of the procs I've looked at only have one
>                 USB interface.
>                 I'm guessing the thing to do here is to use a USB
>                 switch.  Anyone have
>                 some insights?
>
>             many CPU'S have a so called USB-OTG (on the go) interface
>             that allows to
>             switch roles between host and device.
>
>             normally switching to host mode is done by inserting a
>             cable that has
>             the ID pin (the 5th pin on a USB mini/micro connector)
>             grounded, but it
>             can of course also be done under SW control.
>
>             look at e.g. the STM32F105, it has a: "USB 2.0 full-speed
>             device/host/OTG controller with on-chip PHY that supports
>             HNP/SRP/ID"
>
>         Check out the Texas Instruments EK-TM4C1294XL LaunchPad (about
>         US$20).
>
>         It has an ARM Cortex-M4 with USB OTG and good source code
>         examples. They have examples of Host firmware and examples of
>         Device firmware. The documentation makes a big deal of the
>         fact that none of the examples will switch between Host and
>         Device for you, so I imagine the solution is to read the OTG
>         input pin and then "reboot" into the other mode when it
>         changes. It doesn't seem like it will be too difficult. I've
>         tested their USB Audio Class Host firmware example, and it
>         works fine. I assume that the USB Device firmware is even
>         simpler, and also functional.
>
>         The EK-TM4C1294XL LaunchPad is also a good example of the
>         circuit needed. The technical manual for that LaunchPad has
>         the full schematic. One interesting component on the board is
>         a chip that controls the current being fed to the Host port to
>         power the external Device. You'll need to keep the power on
>         the A and B connectors separate. It will be challenging if you
>         want to power your board entirely from USB when acting as a
>         Device, and probably much easier if your board is always
>         self-powered. You'll need to be able to read the B connector
>         power on a 5-Volt-tolerant GPIO pin to detect external power
>         unless you're always self-powered. I think that the ground and
>         differential USB data can be shared between the A and B.
>
>         I'm sure there are other options, but I wanted to point out
>         that the TM4C1294 has good examples for the circuits and
>         source for the firmware.
>
>         Brian Willoughby
>         Sound Consulting
>
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>
>
>     -- 
>     --Tim Ressel
>     Circuit Abbey
>     timr at circuitabbey.com <mailto:timr at circuitabbey.com>
>
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>

-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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