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Topic: SIM900 TXD and RXD Voltage Question Please Help (Read 1 time) previous topic - next topic


Dec 04, 2014, 11:39 pm Last Edit: Dec 04, 2014, 11:43 pm by jacob84401
I am looking to use the SIM900 module.  I see that the module's power supply is from 3.2 V to 4.8 V.  I will be running it with an ATMEGA328P microprocessor who's power supply range is from 1.8 V to 5 V.  Naturally I would like to choose a power supply between 3.2 V to 4.8 V and simply use one power supply.

If this works that would be simplest.  The question comes in when I see that the SIM900's RXD port needs to be pulled up to VDD_EXT externally.  Which is at 2.8 Volts.  Does that mean that serial communication can only work at 2.8 volts and that I cannot use a serial communication at 3.2 to 4.8 volts?  I would like to know before I send data through at 4 V and destroy the SIM900.

I have seen some level shifter schematics which sound like it would work but I would like to avoid that complexity if possible.  If I had to use a level shifter that would mean that I would be using two different power supplies and that would not be optimum. See the attached.

Here is the SIM900 datasheet if needed.  http://itead-europe.com/pm/platform/shield/icomsat/DOC_SIM900_Hardware%20Design_V2.00.pdf

Thanks in advance.

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