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Topic: Using GPRS Shield and RTC simultaneously on Stalker v2.3 (Read 2535 times) previous topic - next topic


Hi everybody

I am currently working on a device that logs incoming phone calls, and sends data to me via text message. I am using a Seeeduino Stalker v2.3 and a SIM 900 GPRS Shield v1.4.

One of the reasons I use the Stalker is because of it's nifty onboard Real Time Clock, which works great. I've also mastered controlling the GPRS Shield pretty well.

Yesterday I was unpleasantly surprised when I tried to use the two simultaneously. The GPRS Shield somehow prevents I2C communication with the RTC.

So I did some reading, and the only (relatively) clear guidance I could find was here:
( 2nd question from the bottom )

Here they claim I can simply cut the SDA and SCL pins, and it will sort out the problem. Apparently the shield doesn't use these pins at all. Is this true?

Before I perform irreversible surgery on my GPRS Shield, I'd like to have some more experienced opinions.
My Stalker and Shield are both newer versions than mentioned in the above website.

So, should I cut those pins?


From their GPRS shield wiki:

Note: A4 and A5 are connected to the I2C pins on the SIM900. The SIM900 however cannot be accessed via the I2C .

Yes, it would make sense to me that you would disconnect those pins. I would suggest that instead of cutting any traces that you simply cut/desolder/remove the A4/A5 pins from the GPRS shield. Two little snips and you're done, and if for some unknown reason you need them back you can replace them.


Yeah, I got impatient, so I just bent them out of the way and tested.
It works :)
I will probably cut them later.


Jan 26, 2013, 08:42 pm Last Edit: Jan 26, 2013, 08:44 pm by snugRugBug Reason: 1
Before I bent away these pins, I used them to check if the shield was on. I don't know why it worked, but it did. Now I need a new way to check if the shield is on. Any suggestions?


I have connected pin 17 ( D17(A3) ) to the Uart pin R1 (or RI? I can't see) with a 10k resistor. Now I can read pin 17 for a power status (the pin goes high when the shield comes on).

Let me know if you think this will cause issues.


The sim900 has a RTC built in, bit harder to use but less hardware.

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