SIM868 PIN code problem reset the board

Hi to everyone,

I’m trying to design a board with different sensors(humidity, pressure,
GPS) that send all the data using the SIM868 and we want to send all the information via GPRS.

I’m having a big problem. When I send the PIN with the command AT+CPIN the board turns off. I have tested a lot of things and I can’t find the source of this problem.

At the beginning I thought it could be a power supply problem, but after a few test I think that the problem is in other site, because I tested it using a power supply with 4V and 2A (the maximum that accept the SIM868) directly to the SIM and the problem was still there.
Now I’m using 5V and 2A to power the board, and I have a regulator that limite the voltage to 4V. I have all the capacitors that are said in the design document of SIM868.

Following some posts of this page I have upgrade the firmware (because I thought it could be the problem) but it wasn’t.

I thought I had a problem with the antenna because if I insert the sim’s PIN without the antenna the board don’t turn off as quickly as when I use the antenna. If I connect the antenna it turns off in less than 8 seconds. I’ve tried to use a SIM without PIN code security, but the board is restarting all the time. I think that the problem is not the pin code but the connectivity with the server. I have tried with 2 different antennas so now I think that the problem should be in other place.

Tell me if I’m wrong but it isn’t necessary to configure anything before introducing the PIN number is it? The SIM868 answer all of the AT commands that I have tried but only turns off with the AT+CPIN command.

I can’t do anything without this command so please, if anyone has any idea of what is happening help me :slight_smile:

Any idea is welcomed, even if it sounds crazy.

Thank you in advance and I hope you can help me.

Hello and welcome!

I see things you can try on hardware and software side:


Since the 2A are usually only consumed at fast peaks during communications, some cheap power supplies drop the voltage dramatically and cause the module to reset or malfunction. If you can check the power rails with an oscilloscope maybe you can diagnose this. Otherwise, try increasing the voltage up to 4.2v to give you a little more of a headroom.

Are you sure that your regulator is able to handle 2A without dropping the voltage too much? In my experience, there are no common regulators that are able to drop only 1v and deliver 2A consistently. Usually you need at least 1.25v to 1.5v of voltage difference as a minimum for the regulator to work properly.

How are you communicating with the module? A serial-to-USB converter, Arduino, something else? Are you using a logic voltage converter?


If you don’t send any message, does the module turns off by itself? Or only after sending the AT+CPIN command?

What does the module respond if you send the AT+CPIN? command?

First of all thanks for the answer!


In general I’m using a mobile charger as a power supply (5V and 2A). But after detecting that the board was restarting I have made the test with a very good power supply of one of the labs of my University. I have used the oscilloscope to check the power rails and the only thing odd happens when the board resets (the voltage drop under 3V)

I detected that the regulator could be a problem, because it can’t handle 2A, its maximum current is 400mA. But when I made the test in the lab I removed the regulator and I connect the power supply where it’s supposed to be the exit of the regulator (in this case it’s not necessary the regulator because I can regulate the voltage and the current with the power supply). The board was restarting too, so for that reason I think that the problem is in other site.

Today I have repeated this test selecting 4,2V as you said and I have tried to send this AT command: AT+CBC (so I can see how the SIM868 detect the voltage level) and the answer was: +CBC=0,85,4087. I think using the power supply with the maximums values that SIM868 admits receive an 85% is a bit strange. I have checked it but I haven’t found anything wrong in the power supply’s rails.

I’m using a custom board with a Nordic microcontroller that sends all the AT commands by UART. This UART olnly connects the SIM868’s UART1 and the Nordic.


Depends. If I’m using a SIM card with PIN code if I don’t send any message it doesn’t turn off. Only turns off when I send the message AT+CPIN=xxxxx
If I’m using a SIM without PIN code the board turns off by itself. After turning off it tuns on again and in less than 10 seconds it turns off again.

Depends again. Using a SIM with PIN code it answers, if I haven’t inserted the pin yet, +CPIN= SIM PIN. If I insert the PIN and I’m fast and I send the AT+CPIN? before the boards restart, the answer is +CPIN: READY.
If I’m using a SIM without PIN code the answer is directly +CPIN:ready.

Thanks for your time!

Ok, that should discard any power supply related problems. Just make sure that the upper limit for the current delivery on the lab power supply is at least 2A.

I’m more experienced with the SIM800 and never used the SIM868, but I suppose they might be similar (you should check in the docs from SIMCOM just to be sure). If you are using a pre-made module, you probably don’t have a way to check how the BAT+ and BAT- pins from the IC are connected, therefore the battery reading might not be correct.

What is the logic voltage on the UART pins of that board? Again, from my experience with the SIM800, probably the logic voltage for the UART pins on the SIM868 board might be 2.8v (please verify this on the SIMCOM docs!). Some module manufacturers might say that the pins are “5v logic compatible” but might be relying only on the fact that the IC has built-in overvoltage protection (that is, the pins are “5v tolerant”, not “5v compatible”).

That sounds to me like the IC is doing something automatically as soon as it gets access to the SIM card. Depending on the configuration of the SIM868, it might be trying to send a unsolicited response through the UART port. Does the module reset by itself if you use the SIM without PIN code and is only connected to the lab power supply (all other pins should be left “floating” for this test)?

It sounds like this SIM should work without problems. I think the source of the resets should be somewhere else.

I’m using a board design by one of my partners, we have checked it and the conexions are ok.

It’s 3.3V. I have checked it and it’s okay too.

Yes it also resets. I think that this is because if I use a SIM without PIN code, the SIM868 automatically try to connect with the provider’s network, and then it is when the problem appears.

I have bought a SIM868 development kit to find if the problem it’s in my program, or in the board design.

Thanks for your answers

Well, I’ve checked the Hardware design guidelines for the SIM868, and it effectively has a 2.8v logic on the UART ports. SIMCOM suggests a simple circuit on page 32 of that document that will allow you to connect it to a 3.3v logic as you need.

Also in that document they mention that the recommended range for VBAT is 4.4v to 3.4v.

Great! Yes, usually making a custom board directly soldered to the IC is something reserved for a more final state. You can also try with a more slimmed down pre-made module that can already be used in production.

As I said before I have bought a SIM868 Development board so I can check if the problem is in the design of my board or in the firmware.

I have tested to use my sim card in the development board and when I send the pin the board restars too.

I always try first to send the command “AT” so I can see if the SIM868 is working.

After receiving the answer I send AT+CPIN = xxxx (where xxxx is the correct number).

Is it necessary to configure something first? As far as I know the PIN code don’t need any AT command first, but maybe I’m wrong. I have tried to send first AT+CFUN=1 but it doesn’t change anything.

If it isn’t necessary to send anything first the problem maybe is the SIM card (maybe because it is 4G) but in theory it should works with 2G. I have tried other sim cards and the result is the same, but all of them were 4G. Any idea?

Thanks in advance

I have used 4G SIMs with the SIM800 without problems, so I don’t think it has to do with that.

Can you send a full schematic with all the connections in your circuit? This sounds like it has to be an electrical issue. Have you tried to communicate with the SIM868 using some other microcontroller?

Finally I have found the problem. It was that the longitude of the return current’s rail was too long.

Thanks for your help.


Excellent! Thank you for taking the time to update us with your solution!

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