Using Raspberry Pi 3 GPIO UART

On RPI3, the native UART is used to control Bluetooth Module.

You have 3 choices (don't forget to reboot your Raspberry Pi after):

Option 1: disable Bluetooth module (recommended if you don't use it)

1. Edit /boot/config.txt and add:

dtoverlay = pi3-disable-bt

2. Edit cmdline.txt and remove (if it's still here):

console=serial0,115200

Option 2: Use both Bluetooth with high speed and UART

CAUTION: If you choose a high speed (500MHz), this method can make your processor heat a lot! Don't forget to add a fan and a heatsink!

With this method, the processor clock will be fixed to a slow speed (250MHz) or to a high speed (500MHz?):

Edit /boot/config.txt and add:

enable_uart = 1

This will affect processor performance because it controls the speed of the L2 cache, and there will also be a reduction in analog audio quality. If you opt for the high clock speed (it will really require a fan and a heatsink), to keep the processor performance and audio quality, also add:

force_turbo = 1

in /boot/config.txt

Option 3: Use both Bluetooth with slow speed and UART

1. Edit /boot/config.txt and add:

dtoverlay = pi3-miniuart-bt

2. Edit /boot/config.txt and add:

core_freq = 250

This will affect the performance of the processor. If you prefer to keep higher performance do not add the line core_freq = 250 but rather the line force_turbo = 1 but it requires to use a fan and a heatsink.

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Thanks to Framboise314 for they great article describing theses methods!