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USB power and debug signals on micro USB connector

For a handheld device, powered by a Li-ion cell, I need a charge power input. Most standard chips for charging and powering the device are meant for USB power or simular. Since the current standard charging connector for mobile phones is a micro USB connector, it seems best to use that. It means you can use a lot of standard chargers. Agreed?

For the device, I also need a serial debug connection. It is OK to have the 3V3 UART connection on an internal header. This means the device must be opened for debugging and maybe a slot must be made in the enclosure for longer test sessions with enclosure.

But is it possible and/or advisable to use the spare pins on the micro USB connector to bring out the 3V3 UART signals? It would save a header and make debugging in the enclosure a lot simpler. But it must not lead to damage to the device and or (windows/linux/mac) PC when the device is plugged in to a PC. There must not be any strange behaviour on the PC and preferably there must not be any detection by the OS of USB activity.

You could ofcourse use a real USB/serial connection, but that would mean adding an FTDI chip or a big software effort on the DSP (which has USB hardware). It would also mean every PC user has acces to the debug channel, I'm not sure we want that.

So putting the 3V3 UART signals (TX/RX only) on the micro USB and use a special breakout box (possibly with FTDI chip) for debugging seems the most practical solution for now. Any arguments against it? Any experiences with such a setup? And which signal on which pin for least chance on damage and strange behaviour? I know some USB chargers have shorts or resistors on the datalines, so at least a few protection resistors on the device side are required.

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail) 

To the systems programmer, users and applications serve only to provide a 
test load.
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Stef
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You mean USB and serial at the same time?

It seems usual for USB mouse to also work as a PS/2 mouse with the same lines and a connector/adaptor. But it is one or the other.

I think you should be at least 5V safe. The way to do that with 3V3 lines is well known, especially for slow signals. (I think you only need resistors.)

I believe it is current limiting resistors, as long as the signals aren't too fast.

As I understand it, USB devices requiring more than the default Icc are supposed to negotiate with the host. That is hard to do when the host is just a power adaptor, so there seem to be conventions for the voltages on the data lines that allow devices to pull more current. That is, for example, how iPod chargers work.

You might also want to do that.

-- glen

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glen herrmannsfeldt

Sorry , wrong group!

I'll re-post in comp.arch.embedded

-- Stef (remove caps, dashes and .invalid from e-mail address to reply by mail)

For most men life is a search for the proper manila envelope in which to get themselves filed. -- Clifton Fadiman

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Stef

Sorry, I meant to post this to comp.arch.embedded. Re-posted it there, I'll try to cancel it here. If you can, put any replies over there. But I'll keep reading here for any replies.

No, I just need the USB for power. I want to use the 'spare' lines for serial.

Protecting my device is probably the the easiest problem. But how to keep the PC from sensing there is anything on the USB and emiting a 'connect sound' for instance?

Guess that will work.

There is an 'apple convention' for signalling power need. The only other 'standard' I know is for USB chargers. They should have D+ and D- shorted to signal the device it can draw 1A?

My worry is not about the power. It is OK if my device does not charge (properly) when connected to a PC. The user should use the specified charger. It may be 'nice to have' to allow for charging from a PC. So it is worth to look in to this a little. But I will not implement the proper USB negotiation, as I do not have USB device software running. But maybe with some sensing it is possible to limit the power draw to only 100mA when connected to a PC.

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail) 

This night methinks is but the daylight sick. 
		-- William Shakespeare, "The Merchant of Venice"
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Stef

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