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I have a Pi 3B+. There are a number of resources out there that suggest it is necessary to enable the disable-bt overlay in order to make use of the serial UART on GPIO 14 & 15. E.g., this document says:

disable-bt disables the Bluetooth device and makes the first PL011 (UART0) the primary UART. You must also disable the system service that initialises the modem, so it does not connect to the UART, using sudo systemctl disable hciuart.

I am happily accessing my Pi over a serial console on pins 14 & 15 even though I have not configured disable-bt in my config.txt (and the hciuart service is enabled, although it is not running).

I do have enable_uart=1, which is documented elsewhere in the same document:

enable_uart Enable the primary/console UART (ttyS0 on a Raspberry Pi 3, 4, 400, Zero W and Zero 2 W, ttyAMA0 otherwise - unless swapped with an overlay such as miniuart-bt)...

I'm a little unclear on the relationship between enable_uart and disable-bt. I'm hoping someone can help clear up my understanding.

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It is true that there are many posts incorrectly claiming there is a need to disable Bluetooth. Most of these are by ill informed people with little understanding of serial interfaces.

The simple answer is (almost) NEVER. It doesn't hurt if you don't use Bluetooth. If you have a particularly demanding serial application or need different number of bits it is needed.

See How do I make serial work on the Raspberry Pi3 (PiZeroW, Pi4 or later models)

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