Help for understanding a circuit

Discussion in 'Homework Help' started by nic6911, Nov 20, 2012.

  1. nic6911

    Thread Starter New Member

    Jun 22, 2012
    I am trying to understand a circuit for a GSM module. It's a shield for an Arduino. The datasheet says that it requires 9-20V on the VIN pin.
    The thing i don't understand is in the top of page 1 in the attached PDF. Its' the small pieces of circuit with some kind of FET doing something....
    There is a Vin, which could be 20V if i did not plug it in my arduino and then there is the Vbat, which is the regulated power (4.1V).

    If i did actually put 20V in on the Vin what would then happen with the voltage on the TXD and TXD_VIN?

    I have only used FET's to drive a motor or something like that.
    As i see it the TXD_VIN pin is pulled to the same voltage as the VIN (20v)???

    Hmm.. i really need a helping hand on this one....

    I really don't get it :(