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[SOLVED] SIM900 stupid modem no SIM_VDD

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micro-hopless

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I'm using a SIM900 modem for connecting to a GPRS network, but the stupid modem will not recognize ANY SIM attached to it !

I have double, triple quadruple check the SIM connections but it won't work !.

I have zero volts on SIM_VDD pin, with 10uF cap to ground. The SIM_PRESENCE pin is connected to ground.

Modem responds to all AT commands, and is still in autobauding mode, which I wish to keep.

What is wrong?
 

Are you getting the proper supply voltage at SIM900 modem?
 

4.05V on the dot !

netlight blinks happily.

- - - Updated - - -

VDD_EXT is 2.824V
 

I think you need to increase your voltage.
The let will blink at low voltage also. But sim detection needs some higher voltage level.(Exact 5V)
 

There isn't any voltage drop.

The filter capacitor for VBAT is 3300uF.

- - - Updated - - -

VBAT is 4.05V, not my mcu power supply.

- - - Updated - - -

The datasheet says that SIM900 must be powered between 3.2 and 4.8V, on the VBAT pins.
 

i think u post your ckt diagram and why u use SIM_PRESENCE pin to gnd ?
 

Did you use a multimeter or an oscilloscope to check the voltage drop?
Also see if all of the module's ground pins are connected to GND.

If you can, post the scheme maybe someone can see something you missed - could happen to everyone.
 

sim900 connection diagram.jpg

modem connections

- - - Updated - - -

I've used a scope to check the voltage drop.
 

What is the purpose of C42 to ground on the clock line?
 

Supposedly for EMI protection.

I've removed C41 and C42, but no change.
 

You might be out of luck with a defective module, check with another module if possible.
Other than that I don't see a reason why shouldn't be working
 

All FIVE modules that I have acquired behave the same !

- - - Updated - - -

Anybody has some working pseudo code (sequence of commands, with responses) available, just to verify that I give it the correct commands.
 

means some SIM900_AT command sequence?
 

GOT IT !

The SIM connector pinout was given in an incorrect order in it's datasheet, so even if the footprint was ok, the actual pinning was not !

Now it works.
 

The SIM VCC capacitor (C40) should be 220nF.

Cheers.
 

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