===== UART =====

===== Introduction =====

This page introduces the usage of UART on [[products:sbc:common:applications:gpio:40pin-header|40pin-Header]].

===== UART Information =====

|           ^  UART       ^ PIN      ^ GPIO Name  ^  DT Overlays Node  ^  Device Node  ^
^  VIM1/2   |  UART_AO_B  |  15(RX)  |  GPIOAO_5  |  uart4             |  /dev/ttyS4   |
| :::       | :::         |  16(TX)  |  GPIOAO_4  | :::                | :::           |
^  VIM3/3L  |  UART_C     |  15(RX)  |  GPIOH_6   |  uart3             |  /dev/ttyS3   |
| :::       | :::         |  16(TX)  |  GPIOH_7   | :::                | :::           |
^  VIM4     |  UART_E     |  15(RX)  |  GPIOY_7   |  uart_e            |  /dev/ttyS4   |
| :::       | :::         |  16(TX)  |  GPIOY_6   | :::                | :::           |
^  VIM1S    |  UART_C     |  15(RX)  |  GPIOD_3   |  uart_c            |  /dev/ttyS2   |
| :::       | :::         |  16(TX)  |  GPIOD_2   | :::                | :::           |
===== Enable UART =====

In order to use the UART, you need to enable the UART function via [[products:sbc:common:configurations:device-tree-overlay|Device Tree Overlay]].

<tabbox VIM1/2/3/3L/Edge1>

Edit ''/boot/env.txt'' to add the uart node to ''overlays'' node if it doesn't exist.

Take VIM3 as an example to enable ''UART_C'', you need to add ''uart3'' node to ''overlays'' node if it doesn't exist.

```shell
overlays=pwm_ao_a pwm_f uart3
```

After reboot, you will see the UART device node.

```shell
$ ls /dev/ttyS3
/dev/ttyS3
```

<tabbox VIM3 with 5.15 kernel>

Edit ''/boot/dtb/amlogic/kvim3.dtb.overlay.env'' to add uart node to ''fdt_overlays'' node if it doesn't exist.


e.g. Enable ''UART_C'', you need to add ''uart3'' to node ''fdt_overlays'' if it doesn't exist.

```shell
fdt_overlays=uart3

```

After reboot, you will see the UART device node.

```shell
$ ls /dev/ttyS3
/dev/ttyS3
```

<tabbox VIM3L with 5.15 kernel>

Edit ''/boot/dtb/amlogic/kvim3l.dtb.overlay.env'' to add uart node to ''fdt_overlays'' node if it doesn't exist.


e.g. Enable ''UART_C'', you need to add ''uart3'' to node ''fdt_overlays'' if it doesn't exist.

```shell
fdt_overlays=uart3

```

After reboot, you will see the UART device node.

```shell
$ ls /dev/ttyS3
/dev/ttyS3
```

<tabbox VIM4>

Edit ''/boot/dtb/amlogic/kvim4.dtb.overlay.env'' to add uart node to ''fdt_overlays'' node if it doesn't exist.


e.g. Enable ''UART_E'', you need to add ''uart_e'' to node ''fdt_overlays'' if it doesn't exist.

```shell
fdt_overlays=uart_e

```

After reboot, you will see the UART device node.

```shell
$ ls /dev/ttyS4
/dev/ttyS4
```

<tabbox VIM1S>

Edit ''/boot/dtb/amlogic/kvim1s.dtb.overlay.env'' to add uart node to ''fdt_overlays'' node if it doesn't exist.


e.g. Enable ''UART_C'', you need to add ''uart_c'' to node ''fdt_overlays'' if it doesn't exist.

```shell
fdt_overlays=uart_c

```

After reboot, you will see the UART device node.

```shell
$ ls /dev/ttyS2
/dev/ttyS2
```

</tabbox>

===== Demo Source Code =====
```c uart.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <termios.h>

// set correct serial path
#define SERIAL_PORT "/dev/ttyS2"

int main() {
    int serial_port;
    struct termios tty;
    
    // open
    serial_port = open(SERIAL_PORT, O_RDWR | O_NOCTTY);
    if (serial_port < 0) {
        perror("Error opening serial port");
        return 1;
    }
    
    // Configuration of serial
    memset(&tty, 0, sizeof(tty));
    if (tcgetattr(serial_port, &tty) != 0) {
        perror("Error getting serial port attributes");
        close(serial_port);
        return 1;
    }
    
    cfsetospeed(&tty, B921600); // set correct baud rate
    cfsetispeed(&tty, B921600);
    
    tty.c_cflag |= (CLOCAL | CREAD);
    tty.c_cflag &= ~PARENB;     
    tty.c_cflag &= ~CSTOPB;     
    tty.c_cflag &= ~CSIZE;
    tty.c_cflag |= CS8;         
    
    // write configration to serial
    if (tcsetattr(serial_port, TCSANOW, &tty) != 0) {
        perror("Error setting serial port attributes");
        close(serial_port);
        return 1;
    }
    
    // loop test
    while (1) {
        // receive data
        char received_data[64];
        if (read(serial_port, &received_data, sizeof(char)*64)) {
            // get data
            printf("Received: %s", received_data);
        }

        // Only send "Hello, UART!"
        char data_to_send[] = "Hello, UART!\n";
        write(serial_port, data_to_send, strlen(data_to_send));
        
        // sleep 100ms
        usleep(100000); 
    }
    
    // close
    close(serial_port);
    
    return 0;
}
```

Compile test code:

```
$ gcc uart.c -o uart
```

===== Test demonstration =====

Run ''uart''

```
$ ./uart
```

After connecting PIN15, PIN16 and GND to PC, open ttyUSBX and input hello

```
1 hello
2 Hello, UART!
```

Check output data:

```
$ ./uart
Received: hello
```

===== Disable UART to Use GPIO =====

If you want to use normal GPIO instead of UART, you can remove the UART node in [[products:sbc:common:configurations:device-tree-overlay|Device Tree Overlay]].