# A circuit to prevent parasitic powering of FT232RL by MCU, will it work?

In a previous question I asked how to prevent the MCU TX pin (3.3V when idle) from powering an FT232RL Usb to Serial chip via the TX line:

Preventing parasitic powering of FT232RL by ATMega328P

I ended up going for a solution where I brought TX and RX low if no more valid communications were detected within a certain time.

This however is still not satisfying. I don't wan't the FTDI chip to be powered at all if no USB voltage is present. Now I have created following circuit:

FTDI3V3 is the 3.3V output from the FTDI LDO that is powered by USB. Switch SW1 simulates the USB power coming on (halfway through the simulation). MCU_TX is a square wave simulating TX activity, and FTDI_RX is the input into the FTDI chip. This should be 0V when SW1 is open.

Simulating the circuit gives the desired behaviour:

• Is there something I have missed?
• Does the circuit adequately simulate the connection?
• Is there an easier alternative? Would a dual supply buffer work?
• Is there an easier alternative? Optocoupler. – Passerby Dec 5 '14 at 6:03
• @Passerby yea that would be ideal, but one for cheap plus low power? So far I've seen some with as low as 40uA current, but the entire board is running at ~90uA in sleep. TX pin is always high so driving that 40uA will increase power consumption a lot. I should have included that in the question. – geometrikal Dec 5 '14 at 9:06
• Your previous question indicates that you Tristate the pin and the ft232 still powers on? Something else might be the cause – Passerby Dec 5 '14 at 13:59
• @Passerby tristate TX and FTDI is off. But when TX is on the FTDI still half powers up which seems 'wrong'. Looking for the more elegant solution. I just built the circuit above (minus R1 - it is not neccesary) and it seems to work! – geometrikal Dec 5 '14 at 22:15