user45551: I need to connect a Raspberry Pi and an Arduino Uno through serial communication and decided to follow this guide, which mentions the use of a voltage divider. 
I understand the connection to the Raspberry Pi GPIO requires 3.3V but I also plan to power the Raspberry Pi from the Ardunio's 5v output pin.
What's the safest approach to achieve this and the recommended current? Thanks.

user3879: 
You CAN NOT reliably power any model Pi from the Arduino 5V output.
You can interconnect the Arduino serial to the Pi using a voltage divider or (preferably) a level converter, as suggested in the linked article. (I might add that the linked article contains a number of errors which will not work on recent Pi models).

Better still you can use a USB connection, to provide serial communication to the Arduino, AND you can power the Arduino from the Pi (I often do this).

45551: <p>I need to connect a Raspberry Pi and an Arduino Uno through serial communication and decided to follow <a href="https://oscarliang.com/raspberry-pi-and-arduino-connected-serial-gpio/" rel="nofollow noreferrer">this guide</a>, which mentions the use of a voltage divider. </p>

<p>I understand the connection to the Raspberry Pi GPIO requires 3.3V but I also plan to power the Raspberry Pi from the Ardunio's 5v output pin.</p>

<p>What's the safest approach to achieve this and the recommended current? Thanks.</p>
 3879: <ol>
<li><p>You <strong>CAN NOT</strong> reliably power any model Pi from the Arduino 5V output.</p></li>
<li><p>You can interconnect the Arduino serial to the Pi using a voltage divider or (preferably) a level converter, as suggested in the linked article. (I might add that the linked article contains a number of errors which will not work on recent Pi models).</p></li>
</ol>

<p>Better still you can use a USB connection, to provide serial communication to the Arduino, <strong>AND</strong> you can power the Arduino from the Pi (I often do this).</p>
