SIM900 start up problem

hello.

I am working with SIM900 module. the schematic of my circuit is here
I turn on SIM900 by applying a low level voltage to the PWRKEY pin. after that, the STATUS pin doesn't go high and the NET pin is always off.
and also SIM900 draws about 120 mA current, that is not normal.

In my circuit, i have also connected the NRESET pin of SIM900 to PD7 pin of the microcontroller.
At first, i configured this pin as output with logical 1 state, and the STATUS and NET pins were ok. But now the module is not working. I thought there may be problem with the NRESET pin, so i configured PD7 as tri-state input, but the problem didn't solved. I also cut the connection of NRESET pin to microcontroller, but it didn't solved.

Is it possible that SIM900 has stopped working?
thanks a lot.

I would check the following:

TX, RX and NRESET have level translation as the SIM900 pins max voltage is about 2.8 volts something and if your MCU is 3.3 or 5 volts this will cause improper operation and or SIM900 failure.

Cheers.

Batang said:

I would check the following:

TX, RX and NRESET have level translation as the SIM900 pins max voltage is about 2.8 volts something and if your MCU is 3.3 or 5 volts this will cause improper operation and or SIM900 failure.

Cheers.

Click to expand...

thanks for your reply.

so can i do this level translation by a resistive voltage divider?

now I have checked the circuit and found that NRESET pin have zero voltage all the time, although i cut the connection of NRESET pin to microcontroller. and STATUS and NET pins also have zero voltage. So is it possible that sim900 has stopped working?

thanks a lot.

I have used resistive dividers with SIM900 with no issue as follow:

MCU TX pin to SIM900 with divider.
MCU RX pin to SIM900 direct connection.
SIM900 Reset pin driven by open collector/drain. Also include 100nF bypass cap on reset pin.

At a guess I think your SIM900 may be damaged.

Cheers