Help for understanding a circuit

Thread Starter

nic6911

Joined Jun 22, 2012
24
Hi.
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 :(
 

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