What is DEBUG pin in debug UART connector?

Started by anverx, May 31, 2016, 12:37:44 PM

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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?


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

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


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.


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


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.


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.


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.