[SOLVED] SIM908 DTR,DCD is always high(3v level) without connecting to MCU

[Karthik_d]

After power on of SIM908-C, The simcard_VCC should have 3V which is supplied to sim card. But it is 0V. I did not connect SIM908-C(serially) with MCU so far. When i checked DTR and DCD, it's showing high(2.8V). But, DTR in SIM908 modem is input pin.

How can DTR have 2.8V without connecting to MCU?(DTR line is to be controlled by MCU only.)
Because DTR is high, i think the SIM908 module goes to sleep and so the simcard_VCC is 0V.(NET light is Blinking).
Please help me.:-(.

 

[wael911t]

Hello There,

If DTR is left floating and not connected, an you are only using TXD - RXD are used then it has no effect, to activate the module you need draw PWRKEY pin low for at least 1 Second ( it is already pulled up internally ) then Pin Status would appear high and SIM VDD will go high either 1.8 VDC or 2.8 VDC depending on the SIM type

 

[Karthik_d]

Yes I did POWER ON module as you said.. BUT the SIM VDD(PIN 17) does not go high.. I haven't used the SIM PRESENCE pin(left floated).. does this have any effect?

 

[wael911t]

small correction; Pin 17 is for SIM IO and SIM VDD is pin 20, anyhow SIM_VDD goes only high once SIM card is inserted and module is power ON, if the SIM is not there SIM VDD goes monetarily high ( about 150 ms ), activate SIM clock for the same period, send some data commands on the Pin 17 SIM IO and if there is no response SIM VDD goes low, this happens only at power up

 

[Karthik_d]

I'm Using sim908-C, the pin connections are attached. I haven't used SIM PRESENCE. How will the module know when i insert SIMCARD?
In the hardware design manual the sim interface connections are given.. what is VDD_EXT on sim presence line in the interfacing diagram?

 

[wael911t]

I have used the SIM908-C along the development board to evaluate the product and test the the AT+ commands, then designed PCB Board with AVR uC and then I faced the same issue you are facing right now, when I digged deep to investigate why the module is not able to register with network, I discovered that Module is not able to communicate with SIM, I asked SIM support about the issue they told me to follow the H/W design reference; however I discovered that IO should be pulled up to SIM_VDD not VDD_ext which is the voltage supplied to the module. also the 22 pF is causing issues with signal shap so I removed and it did work. Regarding the SIM presence pin; it is not mandatory to connect it to pin 8 of the SIM module you can leave it float, you run the module with in 6 Pins mode ( default ) or ( 8 Pins mode ) where you need to connect the SIM presence pin. in 6 Pins mode ; once the Module powered up, SIM_VDD is asserted to 1.8 VDD, SIM_CLK is activated and SIM sends some commands over IO pin, if there is no response from the SIM it repeats that for 3 times each time for around 10 ms, then SIM_VDD is asserted to 2.8 VDD then repeats that for another three times. once SIM is not discovered then SIM908 will work offline, which means you can communicate to it over UART, run some configuration but you cannot send use the other features

 

[Karthik_d]

Thanks wael911t. I found your replies helpful. I figured out the problem, it was 22R Resistor i used in series with SIM_CLK. Now SIM is working fine. And i found that the SIM_Presence PIN needs VDD_EXT because, it comes to contact with CCIN(presence) and CCIO(gnd) pins of 8-PIN SIMCARD holder when the SIM_CARD holder tray is pushed in.
When the SIMCARD tray pulled out, SIM_Presence losts it contact with CCIN and thus it can be used to know the presence of simcard tray and not the SIMCARD.