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Sim908 module power on issue

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Member level 2
Jan 28, 2015
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Hi all,

I'm new to the forum and would be glad if somebody could help me out ASAP.

I've made my own PCB design for SIM908, referring to the datasheet and have been using it in my various several projects for over a year and found no problem. The input to the board is 12V/2A. The output of the LM2576 to the VBAT is 3.98V.

According to SIM908, the PWRKEY has internal pull-up and is always high (3.98v).

Now, my problem is, recently i happened to finish up a board with a MCU and SIM908 (with the same hardware that i used before) and it worked perfectly for almost one week untill i happened to restart it as i was switching my place. After i powered it up again, nothing happened. The N/W led did not blink. The module is still good, but the voltage on the PWRKEY is 1.06v. I keep on receiving junk data from its TX/RX pins. The MCU is working perfect, only my SIM908 module isn't powering up.

What could possibly the problem? Somebody please help me out.

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the internal pull up is 100K which is very high, I'm pretty sure you have a hardware-related issue.

- try cleaning the pcb tracks for this PIN (specially from the transistor to PWRKEY), and also check that the transistor isn't triggered by more dirt on your PCB...

- add an external pull of 10K or 4.7k It will do no harm and maybe pull up for unnecessary debris on the track...

- how are you sure the module is OK? you said that it sends junk data, how is that OK?

-finally, remember that this module is very tricky at the power supply, if it just turns off after some Tx operation (during or initialing a call or sending a SMS) it also could be a instant drop on the VBAT supply... check your regulator, it's inductor and test drawing them at least 2A and expect a very low drop from your 3.98...

Thank you very much for your reply Kurenai_ryu.

- I've cleaned up the pcb several times using IPA pcb cleaner. There is no shorts in any other tracks as well.

- I do not use transistor to turn the module ON. I short the STATUS and the PWRKEY pins, with the PWRKEY grounded by a 470uF capacitor so that, everytime when i plug in the power supply, the module turns on automatically, rather i don't have to trigger the transistor everytime to turn ON the module. I've been using it in all my SIMXXX modules (which are all still working good). Tell me if it is wrong and please tell me if there are any ways to automatically turn on the module once the power supply is plugged in.

-I tried the external pull up using 10k. For the 1st time the n/w led was always high, didnt blink. So, i removed the pull up and again gave the pull up. now the n/w doesnt go up.

- If the module is out, then the VBAT and GND pins are shorted when power is off. Thats how i check if the module is still good or out.

Thanks again.

Please check your power supply voltages. Because the module has its own power sense it will not work if the power crosses or dropped beyond its working levels.

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