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USB power and debug signals on micro USB connector

(you may also find this post on comp.arch.fpga, where I posted by accident. I'll try to cancel it there.)

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) 

Whatever is not nailed down is mine.  Whatever I can pry up is not nailed down. 
		-- Collis P. Huntingdon, railroad tycoon
Reply to
Stef
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You can always add a switch to enable debug. It could be as simple as a pin. Just insert the pin to connect one of the USB & Serial line. I don't think its too much to ask for having a pin to debug it.

Reply to
me

Put a header on the board that provides power, USB signals, and your micro's serial pins. Then build a little mezzanine card with an FTDI chip that makes it all go when it's plugged in.

Without the mezzanine card, the USB port is power-only. With the mezzanine card, you've got your USB-serial.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com
Reply to
Tim Wescott

That is a practical and risk free approach. And I can combine it with Linnix's switch option. Put a small dual row header with the signals on the board. If I plug a card with FTDI chip on, I have direct USB/ serial out. If I put jumpers on it, I get the raw serial lines on the USB connector pins. And if I need that to be a bit fancier, I can put the pin-switch across the header instead of the jumpers.

No thoughts about connecting the serial lines to the USB connector directly, always, in a safe manner? Would save me the above header.

Anyone ever seen 1.27mm shorts (jumpers)? Smallest I can find are the 2mm ones. There are plenty of 1.27mm dual row headers, but so far I did not find the matching jumpers.

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

Youth is such a wonderful thing.  What a crime to waste it on children. 
		-- George Bernard Shaw
Reply to
Stef

Are these what you are thinking of ?

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Simon.

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Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP 
Microsoft: Bringing you 1980s technology to a 21st century world
Reply to
Simon Clubley

Yes, that is exactly what I meant. Thanks for the link!

Had been looking for them a while ago and did not find any. Strange, because Farnell is usually the first place I start looking for such parts. The have those in black/red/blue and with/without handle. Must have missed them somehow, or are they new?

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

The worst part of valor is indiscretion.
Reply to
Stef

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You are welcome.

I don't know if they are new because I don't use them; the smallest size I use is 2.54mm. I just did a search at Farnell for "1.27mm pitch jumper" and picked one result at random out of the 6 returned. However, since you have obviously done the same, I am assuming they are indeed new.

Simon.

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Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP 
Microsoft: Bringing you 1980s technology to a 21st century world
Reply to
Simon Clubley

2mm

not

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parts.

missed

I think these jumpers which you have picked up are very well designed indeed. I have been looking for such jumpers for a while. What I used to do instead is take 1.27 mm female type sockets, cut them in pairs to look like jumpers, then cut the pins from the back, and then solder the pair together to get a very small jumper for the 1.27 mm male type as the result.

I hope this helps.

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Reply to
tasbihmr

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the same part (black with handle) is available here from Digikey at $0.34 qty 1. Unless I'm off on my conversion rates, that's significantly less than the 0.32 pounds on the Farnell site.

Digikey lists them as 1.27mm shunts. They must be fairly new as the quantity prices don't drop nearly quickly as those for 2mm shunts. You can get 2mm shunts at 100 for $7.43. A hundred of the

1.27mm is $22.30.

Mark Borgerson

Reply to
Mark Borgerson

The current exchange rate is roughly about 1.60 USD to 1 UKP so yes the Digikey price is cheaper.

I don't recall ever seeing them been called shunts before, but only jumpers.

Is this another US/UK divided by a common language thing ? :-)

Simon.

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Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP 
Microsoft: Bringing you 1980s technology to a 21st century world
Reply to
Simon Clubley

Could be. However, the Digikey search engine finds them with either descriptive word, so I think the term 'jumper' is widely used here also.

Mark Borgerson

Reply to
Mark Borgerson

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