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OpenRex - Isolate MOSFET Buffers for UART1

Kostas_P , 08-22-2021, 04:54 PM
Hi Robert,

I have purchased an OpenRex about 2 years ago and recently i have been studying the schematics in order to understand all the details on the board.

I would like to ask a question regarding to the the isolate MOSFET buffer ( part Q8A) on page 24 of the OpenRex schematics.

When power is applied to the board, 3.3V is generated and fed to the gate of Q8A. How does the Q8A MOSFET turns ON? The way i understand it (please correct me if i am wrong) is that VGS is 0V because the gate voltage is 3.3V and the source voltage is also 3.3V due to the R370.

I understand the circuit on Q11B but i struggle to understand the circuit on Q8A.

It would be highly appreciated of you can help me on this.

Best Regards,
Kostas_P , 08-22-2021, 04:57 PM
robertferanec , 08-23-2021, 02:15 AM
This is used to prevent current leaking - it is used to isolate CPU signal from the device connected to connector.

We had random problems caused by currents coming from PC (FTDI cable) - the problem was, that some boards randomly didn't boot up in low temperatures and we found out, the problem was caused by leaking currents through FTDI cable (when cable was disconnected, all boards were booting up with no problems).

So we used these transistors which only can pull signals low, they can't pull high (there needs to be an external pull up connected to the local power). Hope this helps.
Kostas_P , 08-23-2021, 04:50 AM
Hi Robert,

Thanks for your prompt reply and clarification.

Could i use a tri-state buffer instead of MOSFETs? or i need to use MOSFETs that include the reverse diode?
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