Do I have this right? P-channel MOSFET to control signal flow

Thread Starter


Joined Dec 17, 2011
Attached is a partial schematic. I just want to know if I have this right. You see two p-channel mostfets (bss84) with their gates connected to the VCC of an ISP programmer. Sources are connected to the serial connector for communicating to an external device. Drains are connected to the serial pins of this device.

What isn't clear is that this device is powered normally (when not being programmed) by +5v (distinct from VCC)

The idea... I think... is that when the programmer is connected and VCC is powered, the external device is disconnected from this device so as to not interfere with the programming procedure. When VCC is not present (programmer is not there and/or not powered) then data can flow to/from the external device as though the fets were not there.

Do I have that right? Does one of the FETS have to have its S/D swapped due to directionality of the signal? Or does that not matter?




Joined Mar 14, 2008
Yes, if the P-MOSFET gate is at 5V then any signal between 0V and 5V would be blocked. If the P-MOSFET gate is grounded then any positive signal above its threshold voltage will pass.

MOSFETs are bidirectional so will pass a signal in either direction when ON.