GSM modem at ext port experiment.


mclien

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Here is the idea:
I want to use my pandora to send sms. (from cli)
So I got this:
And I learned, I shoud use a levelshifter on the RX/TX signals, guess this will do:
Modul itself shoud be povered directly from the LiPoly,as the ext connectors voltage is to low.

-Feel free to point me to a GSM modul which will operate with the ext-connector directly. I didn't found one.

The final setup should be this:
By using a shorter Battery of the same capacity, placing the modem inside the battery compartment and connect it to the internal ext port pins on the mainboard. An external antenna should fit inside the case beside the stylus or in the battery compartment, too.

For testing, I intend to use the ext connector at the back of the pandora, of course.

Information I am lacking so far:
- are those ext serial pins sufficient to communicate with the modul?
(or more accurate: are those addressable as /dev/ttyX ?)
- are there any CAD files of the Battery lid (in case I need a modified one later on)
- ok it seems the levelshifter needs stable input voltage for the hight and the low side. so a regulated source for 1.8 and 3.3 V is needed to make tis working? are those available somewhere on the board?

It is in no way a thoroughly thought through concept, but if anyone is out to help me get this running, I at least would spend a second bundle of components in as a reward (including a modified 3D prind of the lid)
 
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Some Pandora information for UART ports:

Extra link to case stuff: https://web.archive.org/web/2015090...page/articles.html/_/development/openhardware

Possibly you can use Gammu for the SMS stuff.

For level-shifting: https://eu.mouser.com/ProductDetail/Texas-Instruments/TXB0104DR https://www.ti.com/lit/ds/symlink/txb0104.pdf
The TXB0104DR supports 4 bi-directional level shifting, 1.2V->3.6V on both sides. Pandora has 1.8V on EXT3.
 
If I'm not mistaken, the level shifting principal is quite the same for either the sparkfun breakout and the TI chip:
- both need a stabilized supply/reference voltage for both logic levels.

So the question is: are those available for 1.8V (ext con) and 3.3V (gsm logic) or do I have to add 2 step-down converters (one for each voltage)?

Thanks for the link to the hardware stuff.
What I do miss (so far) is the internal PCB of the battery. The contacts it self are easy enough to reverse engineer, but is the must be at least some circuit for the temp(?) sensing. Apart from the usual protection circuit, which is mandatory for every LiPoly battery.
 
If I'm not mistaken, the level shifting principal is quite the same for either the sparkfun breakout and the TI chip:
- both need a stabilized supply/reference voltage for both logic levels.

So the question is: are those available for 1.8V (ext con) and 3.3V (gsm logic) or do I have to add 2 step-down converters (one for each voltage)?

Thanks for the link to the hardware stuff.
What I do miss (so far) is the internal PCB of the battery. The contacts it self are easy enough to reverse engineer, but is the must be at least some circuit for the temp(?) sensing. Apart from the usual protection circuit, which is mandatory for every LiPoly battery.
From my understanding from the logic level shifters I played with in the past. you do need to supply the voltages for each side 1.8V/3.3V. I believe the EXT connector does have a voltage output, but it defaults to 2.8V. It can be programmed according to the Wiki, but that looks a bit problematic.

 
From my understanding from the logic level shifters I played with in the past. you do need to supply the voltages for each side 1.8V/3.3V. I believe the EXT connector does have a voltage output, but it defaults to 2.8V. It can be programmed according to the Wiki, but that looks a bit problematic.

I think the GSM module probably accepts 2.8V so that would be okay.
 
I think the GSM module probably accepts 2.8V so that would be okay.
The issue is the Pandora Digital I/O logic on the EXT connector is 1.8V, yet the output Voltage on the EXT connector is 2.8V by default, so level shifting of some form is needed. And yeah the 3.3V logic on the GSM module likely could handle 2.8V.
 
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So the least problematic solution, since I plan to place all this in the battery compartment, seems to me:
power directly from the LiPoly
2 stepdown moduls (1.8V/3.3V output)
1 logic level shifter (the sparkfunk has 4 channels)
the GSM modem.

I can already place that in the spare bottom part of the case, connect it with the ext connector to the pandora. If I get it working, I can use the cable to the ext connector to wire it directly to the board.

The missing part so far is the circuit which is part of the LiPoly.
 
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