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87 lines
3.1 KiB
Markdown
87 lines
3.1 KiB
Markdown
# UART interface
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UART is an asynchronous serial interface for data transfer that is used in many devices. For example, GPS antennas, Wi-Fi routers, or Pixhawk.
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The interface usually contains two lines: TX for data transmission, and RX for data reception. It usually uses the 5-volt logics.
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To connect two devices, you have to feed the TX line of the first device to the RX line of the other one. A similar manipulation is required on the other end for ensuring two-way data transmission.
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It is also necessary to synchronize the voltages – connect the ground on two devices.
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Read more about the interface and the Protocol in [this article](https://habr.com/post/109395/).
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## Linux TTY
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In Linux, there is the concept of POSIX Terminal Interface (read more [here](https://en.wikipedia.org/wiki/POSIX_terminal_interface). It is an abstraction over the serial or virtual interface that allows several agents to work with the device simultaneously.
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An example of such abstraction in Raspbian may be `/dev/tty1` – the device for text output to the screen connected via HDMI.
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## UART on Raspberry Pi 3
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Raspberry Pi 3 has two hardware UART interfaces:
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1. Mini UART (`/dev/ttyAMA0`) – uses the timing of the RPi graphics core, and therefore limits its frequency.
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2. PL011 (`/dev/ttyS0`) – the full-fledged UART interface on a separate chip of the microcontroller.
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Read more about UART on Raspberry Pi in the [official article](https://www.raspberrypi.org/documentation/configuration/uart.md).
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Using microcontroller valves, these interfaces may be switched between two physical outputs:
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1. UART connector on GPIO;
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2. RPi Bluetooth module.
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By default, Raspberry Pi 3 PL011 is connected to the Bluetooth module. And Mini UART is disabled with the value of directive `enable_uart`, which is `0` by default.
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> **Note** One should understand that directive `enable_uart` changes its default value, depending on which UART is connected to the RPi Bluetooth module with directive `dtoverlay=pi3-miniuart-bt`.
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For the sake of convenience of working with these outputs, aliases exist in Raspbian:
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* `/dev/serial0` – always points to the TTY device that is connected to the GPIO ports.
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* `/dev/serial1` – always points to the TTY device that is connected to the Bluetooth module.
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### Configuration of UART on Raspberry Pi 3
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To configure UART, there are directives located in `/boot/config.txt`.
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To enable the UART interface on GPIO:
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```txt
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enable_uart=1
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```
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To disconnect the UART interface from the Bluetooth module:
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```txt
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dtoverlay=pi3-disable-bt
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```
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To connect Mini UART to the Bluetooth module:
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```txt
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dtoverlay=pi3-miniuart-bt
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```
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If the Bluetooth module is disabled, one should disable the hciuart service:
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```bash
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sudo systemctl disable hciuart.service
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```
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## Default image configuration
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In the [Clover image for RPi](image.md), we initially disabled Mini UART and the Bluetooth module.
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Bugs
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----
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If you use the Mini UART connection to Bluetooth, `hciuart` crashes with the following error:
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In case of Bluetooth disconnection
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```txt
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/dev/serial0 -\> ttyAMA0
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/dev/serial1 -\> ttyS0
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```
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