Hi !
I know the raspberrypi can share a max input/ output of 3.3V on its GPIOs.
Can we connect the Telemetry port of Pixhawk to Raspberry pi’s UART directly ?
The ardupilot webpage shows so.
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Yes you can directly connect them.
Agreed, You can. However, I decided in my current RPi project to use protection resistors on all externally routed signals (i2c and uart included). I’m using 220 ohm that will limit the max current (in case of a short to 3.3v when drivel low) to 15mA. My first choice of value was 250 ohm, but the closest I had was 220.
I decided to use those resistors after my last crash during which the pi itself was well protected physically, but another part connected to i2c shorted to 5v and blew every 3.3v device on the i2c bus(pi included) 
. In the current situation where RPi Zero2W are so hard to get breaking one up can be a serious setback.