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Author Topic: What is DEBUG pin in debug UART connector?  (Read 4645 times)

anverx

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What is DEBUG pin in debug UART connector?
« on: May 31, 2016, 12:37:44 PM »
Hi
I'm connecting mod-gps to debug port of oLinuxino Maxi.  UEXT UART is taken by something else. 

When i draw power from UEXT and connect TX/RX to DEBUG TX/RX, i receive the NMEA data and gpsd runs just fine. 

What i would like to do is to have a single 4 pin connector for Debug UART.  Debug UART port consist of 4 pins, TX/RX/Ground/Debug.  I'd like to draw 3v from debug pin to power the GPS. If it's a pin coming directly from the processor it should supply the needed 40mA.  But how would i set the Debug pin to a high state?

Kean

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Re: What is DEBUG pin in debug UART connector?
« Reply #1 on: May 31, 2016, 01:18:53 PM »
Page 32 of the user manual[1] shows it comes from processor pin 84 (a typo, it is actually 89)

Page 36-5 (1407) of the imx233 reference manual list this as the serial JTAG debug port, which is described in chapter 34, more particularly section 34.2.7.  From this you'll see it is not a user programmable output.

Also, note that even on normal I/O pins the maximum drive strength is 12mA, or 16mA/24mA on certain special pins (see 37.2.2.1).

So basically, you can't power the GPS from this single connector - you'll need to make up a Y cable of some sort.

Hope this helps.

[1] https://www.olimex.com/Products/OLinuXino/iMX233/iMX233-OLinuXino-MAXI/resources/iMX233-OLINUXINO-MAXI.pdf
[2] http://cache.nxp.com/files/dsp/doc/ref_manual/IMX23RM.pdf?fpsp=1&WT_TYPE=Reference%20Manuals&WT_VENDOR=FREESCALE&WT_FILE_FORMAT=pdf&WT_ASSET=Documentation&fileExt=.pdf

anverx

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Re: What is DEBUG pin in debug UART connector?
« Reply #2 on: June 01, 2016, 04:40:19 PM »

Thanks for clearing that out Kean.
Guess i'll have to stick to 2 and 2 pin connectors. 
Didn't realize there was so little current in processor pins, i somehow assumed it was around 50mA.
I actually tried flicking GPIO 84 on and off, but i wasn't sure if processor pin 84 is the same thing as GPIO 84, and, as you say 84 is actually a different pin entirely.

anverx

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Re: What is DEBUG pin in debug UART connector?
« Reply #3 on: June 02, 2016, 12:27:00 PM »

It looks like gps data in debug port is confusing the bootloader and device doesn't boot. Dammit.

Kean

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Re: What is DEBUG pin in debug UART connector?
« Reply #4 on: June 02, 2016, 03:16:49 PM »
Yes, I think certain keystrokes at boot could cause problems.
Could you use a spare GPIO to drive a transistor (MOSFET) that turns the GPS power on after boot?
Some GPS modules actually have a separate enable/sleep input that can be driven directly by a GPIO.

anverx

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Re: What is DEBUG pin in debug UART connector?
« Reply #5 on: June 03, 2016, 11:48:07 AM »

I'm using a custom expansion board, which luckily has a spare relay that i can use.  Is there any practical difference which line to run through the relay, GPS power or GPS TX ? 
Cutting the TX makes more sense to me because the GPS can then start together with the board.  GPS cold start takes a long time.  In my application the coordinates are only read once a few minutes, so it isn't very critical either way.

MOD GPS (as sold by Olimex) has only 4 wires in the cable: Power/GND/TX/RX, which makes sense since it has the UEXT connector.

Kean

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Re: What is DEBUG pin in debug UART connector?
« Reply #6 on: June 03, 2016, 02:39:49 PM »
Yeah, if you're not worried about power consumption (and don't otherwise need to restart the GPS), then switching the GPS Tx line seems sensible.