alexchamberlain
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Will the Model A expose the extra USB bus?

Fri Aug 03, 2012 9:12 am

As you can see on page 3 of the schematic, it seems there will be a spare USB bus on the Model A, as the plan is to only have one USB port? Is this correct? If so, will it be exposed on a header? (So that maybe, a USB hub/Ethernet/<insert favourite USB device here> expansion board can be added?

Mods: Wasn't sure which forum to post this in?
Developer of piimg, a utility for working with RPi images.

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AndrewS
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Re: Will the Model A expose the extra USB bus?

Fri Aug 03, 2012 9:59 am

I think you've misread the schematic :|
The BCM2835 chip at the heart of the Pi only has one USB (OTG-capable) bus. On the ModelB boards this is connected to the LAN9512 chip, which provides 2 USB host ports, and a 10/100Mb ethernet connection.
On the ModelA boards it (will be) connected to the single USB host port, so there's no 'spare' bus.

There's speculation that with the USB hub chip 'not in the way', the ModelA's single USB port might be usable in OnTheGo mode (i.e. both host and device).

6677
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Re: Will the Model A expose the extra USB bus?

Fri Aug 03, 2012 9:41 pm

As andy says the pi's main CPU only actually has 1 USB port (capable of host and slave modes) and no LAN capabilities. Instead its got a second chip on board which has 2 USB's (host only) and a 10/100 ethernet port. I assume this is probably connected via the CPU's USB as some sort of hub, probably wrong but thats the first viable method that popped to mind.

Rather than the A having a USB and LAN removed its a case of the foundation having to do work to get a second USB and the LAN on there in the first place

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mahjongg
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Re: Will the Model A expose the extra USB bus?

Fri Aug 03, 2012 9:57 pm

Yes, the model-A is simply a model B with the (USB hub and ethernet-adapter) chip LAN9512 removed, and the USB signals from the broadcom SoC bypassing the LAN9512 Chip using a few zero-ohm resistors to route them directly to the USB port (that has a single port USB connector mounted instead of a "double decker".

Because of the conventional USB model-A connector used the PI won't instantly gain "USB on the GO", but theoretically it possible to use one of the GPIO as a "host/slave" mode signal, or to simply change the fuction of the port from host to slave (but with a cable to change the wiring to slave mode wiring, in slave mode).

alexchamberlain
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Re: Will the Model A expose the extra USB bus?

Sat Aug 04, 2012 4:10 pm

Yeah, I misread the schematics - I'm not an electronic engineer and so even the simplest schematics aren't easy for me to read.

Thanks all for the info. I'd like to see a USB bus on a header in future revs, but I can understand if it wasn't!
Developer of piimg, a utility for working with RPi images.

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