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SIM900 - Solution found to random reseting or not detecting the SIM

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Apr 28, 2007
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I am just sharing some information with the hope that it could be useful to people having this common problem with the SIM900. After fighting with the random restarting of the SIM900 module for couple of days, finally found the problem and I am posting my solutions to that.

PROBLEM1: Power supply drop - This is more common problem and it is often resolved by using more powerful batteries. However there is better solution and the reason is not the bad power source but that if a power inductor with not sufficiently high current rating (few amps at least) is used on the power supply line, as indicated on the reference datasheet, when the GSM module start sending or receiving, because of the inductor there is a drop in the voltage to about minus -0.7v (there is a clamping diode) for 5 to 10 nano-seconds. Unfortunately this is enough to restart the module.

SOLUTION: In my tests keeping the PWRKEY constantly pressed down seems to be enough to prevent the reset in short power drops. However, another possibility that I opted for, is to replace the power inductor with two or three SMD tantalum capacitors of 330-680uF in parallel connected to ground as show on the schematic - the physical size of the two additional caps is the same as a high current inductor. In my tests this removed any transient response drops even with weaker batteries.

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PROBLEM 2: After fixing the power drops I found the real cause in two of the modules that were problematic. For reason unknown to me, sometimes the RESET line on the SIM-card starts oscillating with a frequency of about 7 MHz with no obvious reason - also I couldn't find any information on the Net about that. When the GSM module attempts to use the network, the SIM-card can’t be properly initialized because the SIM RESET pin does not respond as expected and the SIM900 module powers down the SIM_VCC, then the GSM network drops the connection to the module too because there's no detectable SIM card.

SOLUTION: As shown on the diagram to suppress the oscillations I placed a single 100nF capacitor to ground on the SIM RESET line - this resolved the problem and no further oscillations were observed. This is more subtle problem and if you have random resetting, even after fixing the power supply as mentioned, then there is good chance that this fix might help in your case too.

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Those two fixes resolved all issues with the SIM900 connection and no problems were observed after thorough testing.

Hope this information could be useful to you.
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