What is the purpose of the 74AHC1G125 (SINGLE BUFFER GATE WITH 3-STATE OUTPUT) in the schematic and how does it works ?

 

They are useful in making a shared data bus.

 

What is the purpose of the 74AHC1G125 (SINGLE BUFFER GATE WITH 3-STATE OUTPUT) in the schematic and how does it works ?

The 74AHC1G125 is a non-inverting tristate buffer with an active low enable. So the output will follow the input only when the enable pin is low. It looks like the circuit will apply 5v from the USB port unless "VIN" has a voltage applied to it. If VIN has a positive voltage on it, it will prevent the PMOS transistor from turning on.

 

Thank you for your explanation.

 

What is the purpose of the 74AHC1G125 (SINGLE BUFFER GATE WITH 3-STATE OUTPUT) in the schematic and how does it works ?

At first glance, it would appear that it was being used to enable power from the USB input. On closer examination, the P channel MOSFET is wired backwards, so it's always enabled (with an undesirable diode drop).


This is from Sparkfun.com, so either the schematic is wrong, or it doesn't work as advertised.
https://www.sparkfun.com/products/13672