SIM900A - am I being stupid?

Discussion in 'Wireless & RF Design' started by liamcrayden, Sep 1, 2016.

  1. liamcrayden

    Thread Starter New Member

    Dec 3, 2012
    I'm making a project that involves interfacing with a SIM900A to send SMS messages.

    I have purchased the following item:

    The seller is not technical, and is probably just a reseller or distributor - he couldn't answer my question.

    To keep things simple, I've connected the SIM900A directly to the serial port of my PC (using on-board MAX232), but the actual project will be on an AVR. The SIM900A is provider locked to Asian countries, so I have flashed original SIM900 firmware (worldwide unlocked) to the board, which works fine. I can communicate with it no problem, all responses respond "OK", except the SIM card is never recognised. It's worth noting however that I had the same problem before I changed from the original SIM900A firmware, so this is unlikely the problem.

    On power up I receive the following message in the console:

    Code (Text):
    ... which indicates that the SIM card is not detected (regardless of provider lock). I have the circuit powered by 5V 2A supply, more than adequate. I have been reading that the SIM card expects 3V3 to power up.

    Pin 30 of the SIM900 (SIM_VDD) measures 0v. I have looked up the pinout of the SIM connections themselves and directly soldered 3V3 power to the SIM, but this doesn't seem to make a difference.

    Am I being stupid? Should I be providing the 3V some other way? On the eBay listing it says:

    But the board itself only seems to have VCC5. Please help! :(

    Datasheets here: Design_V2.00.pdf
  2. liamcrayden

    Thread Starter New Member

    Dec 3, 2012
    Worked it out. Three UK (Hutchison Telecom) is a 3G/4G only network, there is no 2G capability. As soon as I tried a Vodafone or O2/Telefonica SIM card it connected up straight away.

    How annoying! Hope this helps somebody else.