Cfrtjryk

I'm trying to understand the impact of using UART communication of the Raspberry's GPIO pins. So I'm reading the Raspberry UART documentation, but I'm not sure if I get everything right.

It says:

By default, on Raspberry Pis equipped with the wireless/Bluetooth module (Raspberry Pi 3 and Raspberry Pi Zero W), the PL011 UART is connected to the BT module

I'm not sure what "connected" means in this context. Because it says "by default", it seems to me that there are other options, potentially configurable, so it's just a software connection.

Later, the documentation says

pi3-disable-bt disables the Bluetooth device and restores UART0/ttyAMA0 to GPIOs 14 and 15.

So does "connected" mean that a) the bluetooth data is forwarded/copied to the PL011 UART (one-way)? Or b) does it also mean that writing to the GPIO pins will send data over bluetooth (two-way)? Or c) the connection between Bluetooth and UART on GPIO pins is an exclusive one. You could either use Bluetooth or use UART.

Last, I have a question on the sentence

pi3-miniuart-bt switches the Raspberry Pi 3 and Raspberry Pi Zero W Bluetooth function to use the mini UART (ttyS0), and restores UART0/ttyAMA0 to GPIOs 14 and 15.

in combination with

The particular deficiencies of the mini UART compared to the PL011 are : [...] [long list of disadvantages]

Since I need Bluetooth and UART, this seems to be the right option to me. Does Bluetooth work really fine using the Mini-UART? I definitely don't like to have Bluetooth issues.

The three options are:

default configuration

• UART0/ttyAMA0 to Bluetooth


• Mini-UART/dev/ttyS0 to GPIOs 14 and 15

pi3-disable-bt

• no Bluetooth support


• UART0/ttyAMA0 to GPIOs 14 and 15

pi3-miniuart-bt

• Mini-UART/ttyS0 connected to Bluetooth


• UART0/ttyAMA0 to GPIOs 14 and 15

In general, the Mini-UART has one big pitfall. It doesn't have its own clock source, so the UART bitrate depends on the CPU clock. Which means you have to set a fixed CPU clock for reliable communication. For serial console this is usually not a problem because it's a debugging tool.

(See Janka's answer for the practical implications, I just wanted to clear up another aspect of the question).

so it's just a software connection.

Not exactly.

The BCM283x SoCs like many higher end microcontrollers and SoCs has a pin mux unit. This allows different combinations of perhiperals to be connected to the pins of the SoC.

The BCM283x SoC has two UARTs, the "Mini UART" and the "Full UART". The pin muxing functionality in the SoC can be used to connect either of these UARTs to either the serial pins on the GPIO header or to the on-board bluetooh.