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spirilis

Programmed & POWERED an MSP430 via RESET/TEST

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So I don't know if this warrants a whole thread, but since it just happened I figured I'd share...

 

One of my projects is a remote control for my LED strip water jug lamp, and today I went to reprogram it (seems to be noticing too much noise in the potentiometer & runs down the coin cell battery with frequent nRF24L01+ RF transmits) and it worked as expected; I have GND, TEST and RESET broken out (not Vcc) so only those 3 were connected to my launchpad.

 

I turned off the potentiometer-RF-update feature temporarily while I figure that out, then reflashed the MCU (it's a G2553 TSSOP-20).  Nothing strange so far.

 

...Then I disconnected the jumper wires, turned the board over and realized the battery was missing!  The whole project was powered parasitically through the RESET/TEST lines and programmed just fine.  Guess I could go back and verify that it flashed correctly, but so far the firmware behaves as expected so I assume it's fine.

 

PS- This mainly shows that I need some coffee!

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I think someone else notice something similar. The USB/Debugger part of the launchpad can be parasitically powered over the reset/test jumpers, even if vcc is disconnected. All three must be removed if you don't want to waste power on that side of the board.

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Probably there's a over voltage protection on either pin. This means basically a diode from ground to the pin and a diode from the pin to vcc. Whenever you apply a voltage to either pin, the vcc can not be less than that voltage minus 0.7. So a voltage of 3.6 volts means there is a voltage of 2.9 on vcc through these diodes. Since 2.9 volts is enough to power the msp430, you're up and running. :-D

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Wait ... how exactly would i use such a device considering 2 scenarios

 

I want that 6 pin device in my launchpad and still program it.

I want that 6 pin device in my breadboard and use it.

 

this is quite interesting if i want to minify a project and am incapable/uninterested of creating custom boards and/or soldering small packages :)

 

thanks,

 

-lastaid

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Wait ... how exactly would i use such a device considering 2 scenarios

 

I want that 6 pin device in my launchpad and still program it.

I want that 6 pin device in my breadboard and use it.

 

this is quite interesting if i want to minify a project and am incapable/uninterested of creating custom boards and/or soldering small packages :smile:

 

thanks,

 

-lastaid

Not quite following, but I wouldn't recommend depending on this port pin protection diode feature to power your whole project, I was just surprised it worked (but not too surprised now) :)

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I do realize that it might be unwise to power an led through a port using this setup, but a transistor might work and solve this problem, at least for my use cases [  see http://forum.43oh.com/topic/4262-annoy-a-tron-20/ ]

 

what i was asking for is your suggestion how to connect everything. tst to tst, rst to rst, but where to connect power and where to connect ground? could it just be any pin?

 

thanks

 

-lastaid

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You can use any pin to power the device. Once you flashed your code, just connect any port pin or rst and Vss to your battery and it'll run.

It's all about the current in this case and depends on your LED but with a transistor it should work.

 

The port pin can't be an output obviously.

 

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I also discovered this trick, I connected a capacitor between GND and VCC pin and used two port pins for power and an experimental communication channel. This way it is okay to exchange the lines - they are full-bridge rectified. Do you know this RFID tag ? http://scanlime.org/2008/09/using-an-avr-as-an-rfid-tag/ It is using similiar ideas.
 

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