SIM808 suddenly dead?


I am using the SIM808 in my circuit and I am powering it with 3.8 Volts.

This has been working well for some time, then the modem started sending only pulses of 15.65 ms and pauses of 13.91 ms. It is no more responding and permanently sending the pulses.

I changed the modem just to find this happening again after a few days…

Are the modules really broken? Is there some way to unbrick it?
What can this mean?

Hint: I might not always use the shutdown function of the modem, because if the embedded main controller hangs up, I don’t get the chance.
I had a similar design with a SIM900 which did not have any issues for years now…

Thanks in advance.

how are you providing the 3.8 volts?

by “embedded main controller” do you mean an external MCU interfaced with the SIM808?

those pulses are coming through the serial ports?


I have a stepdown converter and the voltage quality is fine with a minimum ripple (3 mV pkpk)

Embedded main is an external controller with 3V3 and the voltage divider from the SIM808 HW Guide.

The pulses are coming out of the TX Port

Excellent, it sounds like a good setup. Just to check, are you sure your whole power delivery network can provide the 2A peaks required by the module without buckling? Have you tried running the stepdown output at 4.2v for example? At some point I’ve used some cheap stepdowns that buckled a bit, and the solution was to simply increase the voltage closer to the upper limit (which is at 4.4v if I’m not mistaken, so 4.2v should be safe).

Which function are you refering to, exactly? There are a couple, plus some other ways to “reset” the module, which might be similar in function depending on your goals.

Again, just to check… are you refering to the next circuit?

please excuse my late reply.

I am trying to use the CPOWD=1 function usually, although while debugging I can’t be sure it always works (e.g. when main controller locks up)

I am using this circuit

I have opened the bad modem (removed the shield) and it seems there is some green fluid/gel in it which seems strange to me. The only fluids I use are flux and pcb cleaner.

Can’t you check on startup of your controller if the sim808 is accesible and on which state? You can maybe reset the gsm module on startup if you see it’s already on. On a side note, I’ve always just attached together the Reset pins of both, to make shure that both start at the same time (you can even issue a reset AT command which would pull down that pin, effectively hardware resetting your whole system); and I just shut it down via “abnormal power off”, that is by just unplugging the power. No issues in a couple of years of operations (although I use the SIM800 only as a “internet modem” without any of the extra functionality).

Keep in mind that both schematics refer to a direct connection to the SIM808 module (i.e. only what’s inside of the EMI shield) and assumes a logic level of 2.8v for RX and TX. If you have any external electronics on your GSM808 you have to check that the available RX and TX pins are working in the 2.8v logic level of the chip, or if it already has a logic level converter somewhere in there.

How did you determined that the modem is bad? The fluid/gel might be a number of things, from thermal or humidity protection, to just some remnant of the assembly process. I hope you didn’t damage the module by removing the shield, as it might be quite difficult to remove without proper tools. Just don’t expect the module to work properly before you reattach it. The EMI shield almost completely seals the module, so I doubt that you or someone there accidentally managed to put something with a gel consistency in there.