USB - Newbie questions


I am doing a redesign of a product using an AVR and a bunch of other components to use a single AT91SAM7S256.

We currently have 2 external serial ports to connect to a pc (although we can use only one).

So some questions

Is there a USB driver that I can easily use with this chip to create the virtual comport (our device can pretend to be a modem)

Is it possible to have two virtual serial ports over the single USB connection without creating all kinds of new software. Multiple virtual comports would be very useful to us.

Thanks Ralph

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There is a standard USB 'modem' device interface specification. See the CDC class here...

If you write firmware for this device that conforms to this interface, then you won't need a driver at all... caveat follows...

In theory, yes, you can have multiple virtual serial ports over a single USB connection. Practically, you're limited only by the number of endpoints that the chip supports. (Actually, some clever programming and you could in theory layer as many as you want over one or more endpoints, but that's not what you're after).

Some time back I succesfully implemented a CDC port on an EZUSB as a proof-of-concept for a product I was developing for a customer. I could talk serial with Hyperterm with no driver what-so-ever.

However, the windows side of things was a bit flaky and prone to crashing. You are also limited to a *single* function device - the CDC can have *no* other endpoints on the connection - or windows won't talk to it at all! Again, not what you're after.

If you really need multiple serial ports, then I'd imagine you're looking at writing a driver.

FWIW I read plenty of posts at the time complaining about the flaky nature of CDC support under windows. I wasn't overly concerned as it wasn't a requirement for production.

Having said all this, IIRC this was WinXP SP1. I have no idea whether or not this is still an issue in later versions of windows.

Of course, none of these issues apply under Linux. ;)

Regards, Mark

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Mark McDougall

Don't know if your AVR has built in USB. If not check out FTDI:

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They have serial port drivers that will talk to their USB-Serial and USB-Parallel chips. Plus they support Windows, Linux/x86 and MacOS X.

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