SIM900 doesn't stay OFF

[Marius_IMV]

Hello,

I'm designing a board for some GPRS communication. The module works fine so far but if I want to turn it OFF it wont stay OFF and after 10 sec it powers ON by itself. Also I cannot power it OFF from the power key pin, the only way is to send the AT+CPOWD=1 command. After this command it says NORMAL POWER DOWN, the STATUS led goes OFF, also the NETLIGHT stops blinking. After aprox 10 seconds STATUS goes ON, and I receive RDY and so on like normal power ON.

I should mention that I think I made some stupid things when testing the modem like pulling the power KEY pin directly to GND when trying to power it on even if in the HD file was a 1k resitor in series with the transistor. Also when I power it down the voltage on the power Key pin goes to zero (and that is not normal i think - measured with pin left floating).I tried to pun a 10k pull up resistor (when powered down) but the voltage is still zero (like something inside pulls it down). When it's ON the voltage goes to 2,8v (also with pin floating).

Anyone have any idea what is going on and what parts inside the modem that might be broken ? (Just curious ...) I'm happy that it stays On and not OFF because I can do some work but I want to know what I should avoid on the final design.

 

[salmanliaquat]

pulling the power KEY pin directly to GND when trying to power it on even if in the HD file was a 1k resitor in series with the transistor

yes it is recommended but without it, it won't damage the module.

Also when I power it down the voltage on the power Key pin goes to zero

if this means that when you apply high at transistors base.....and since you have not connected series resistor so i think its pretty clear why the voltage is 0.

I tried to pun a 10k pull up resistor

This is meaningless because SIM900's PWRKEY pin is internally pulled up to V_EXT.

When it's ON the voltage goes to 2,8v (also with pin floating)

i think your problem lies here. why is the voltage still 2.8v after the module is ON. you need to apply just a pulse. and which pin is floating ..?

 

[Marius_IMV]

When I said that the pin goes low I meant with the pin left floating without any transistor connected (I powered it OFF with the power off command because by pulling the pin low wouldn't work). If it has a pull up resistor wouldn't still be at high level regardless of the state of the modem's power (because to turn it On again we should get the pin down to zero again)? I mentioned this also in the first post when I said that the pin was left floating.

(TO salmanliaquat: ... why is the voltage still 2.8v after the module is ON ... Because is pulled up and to power it down we need to drive it low like when it is OFF)

 

[salmanliaquat]

You mean you are operating the modem with its PWRKEY pin floating...!!!!!!!! Then how would you expect it to work properly. Remember PWRKEY is an input not an output.

because to turn it On again we should get the pin down to zero again)?

yes. just for 2 secs. then again let it go high.

I would suggest you to connect the PWRKEY pin according to the instructions provided in HD_manual, and then try turning on or off the module by applying pulses at base of transistor.

 

[Marius_IMV]

I tried that in the first place and wasn't working, that's why i started investigating and tries all the stuff that I tried. With the transistor is not working either.

 

[devil6600]

I tried that in the first place and wasn't working, that's why i started investigating and tries all the stuff that I tried. With the transistor is not working either.

hi.. did you got it working? i am also facing same issue..

 

[Marius_IMV]

No, I didn't, I will use a SIM908-C for the final project and I abandoned researching this problem. Interesting fact: It stays OFF after I apply the voltage and I can power it ON only by touching a metal thing on the PWR pin... If I try to power it ON or OFF in the normal way (as in documentation) it doesn't work at all. I think the input is burned somehow ... lets hope I will not encounter any other problems because I already ordered the PCB for the SIM908-C.

Try using the other SIM900 variant (i think is SIM908D) that has 3v IO tolerant pins and battery charging circuit integrated.

 

[devil6600]

thanks for reply. i'll give sim908D try in my next application.
what voltage you were working with sim900? i am thinking to use it at 3.3V. i have already tested at 3.3v, it works, but i am not sure if i can use it in long term.

 

[Marius_IMV]

I use some level translators but it's not a solution (they take a lot of PCB and they are expensive ~2eu/4ch). I have the controller at 3,3v and SIM900 works at 2,8v. Another solution might be to use transistors instead of the level convertors.