Setting up Raspi: UART

So one big addition to this new SLAM robot is the stereo cameras I plan on using to achieve visual odometry. I had a couple webcams I salvaged a bit ago and they were easy to mount onto a piece of aluminum I had laying around. They are both Logitech C270 webcams. They're not the best resolution, but since I plan on using an on board Raspberry Pi 3 to do the processing, the 720p resolution hopefully will allow for fast enough image processing on the Pi.

Stereo webcams on mounting plate.

Before I begin to implement any type of image processing or complex algorithms on the Pi, I must establish a link between the Pi and my microcontroller that will be interfacing with the stepper motors. Right now I am using a Arduino micro for the ease of implementation, however I eventually want to move over to using a PIC18F2550 microcontroller for size considerations. Starting with the Arduino and then porting the code over to the PIC might help some beginners reading this do the same. 

My Pi 3 is running Raspbian Jessie Lite, kernel version 4.4. Note that the following instructions are for the Pi 3, the UART port is changed on the Pi 3 from previous versions due to the addition Bluetooth. If you are familiar with using UART on previous Pi's, the UART lives at /dev/ttyAMA0. This port is now the connection to the Bluetooth controller. The UART on the Pi 3 is now at /dev/ttyS0. If you do not see this show up in your file listing, follow the bellow configuration and it should show up after. 

Configuring the UART:

1) Stop and disable serial service

    sudo systemctl stop serial-getty@ttyS0.serivce
    sudo systemctl disable serial-getty@ttyS0.service

2) Change command line boot options

    sudo nano /boot/cmdline.txt

You will see something similar to console=serial0,115200 in the file, remove this and any other mentions of serial0 and save the file.

Pi 3 header pinout.

3) Enable the UART pins

sudo nano /boot/config.txt

Append enable_uart=1 to the bottom of the file and save.

4) Reboot your Pi.

You should now be ready to use the UART on the Pi 3. To test that the configuration works, I connected the UART TX pin, GPIO14, to my oscilloscope to observe the output. 

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>

int main(int argc, char* argv[]) {

    struct termios serial;
    char* str  = "Hello, world!";
    char* port = "/dev/ttyS0";

    printf("Opening %s\n", port);

    int uartPort = open(port, O_RDWR | O_NOCTTY | O_NDELAY);

    if (uartPort == -1) {
        perror("Failed to open port\n");
        return -1;
    }

    // Set up UART port
    serial.c_iflag = 0;
    serial.c_oflag = 0;
    serial.c_lflag = 0;
    serial.c_cflag = 0;
    serial.c_cc[VMIN] = 0;
    serial.c_cc[VTIME] = 0;
    serial.c_cflag = B115200 | CS8 | CREAD;

    tcsetattr(uartPort, TCSANOW, &serial); // Apply configuration

    // Attempt to send and receive
    printf("Sending - %s\n", str);

    int r = write(uartPort, &str, strlen(str));
    if (r < 0) {
        perror("Failed to write to UART");
        return -1;
    }
    else {
        printf("Sent successful\n", r);
    }
    // Close port
    close(uartPort);
}

Compile the above code with sudo gcc main.c -o main, where main.c is the name of the file the code is in, and then run the compiled code with ./main

The output should look something like

Opening /dev/ttyS0     
Sending - Hello, world!
Sent successful        

The output capture on my scope verifies that the Pi 3 is transmitting our string from the UART. One important note about the waveform, the Pi 3's UART operates at 3.3V, however I have added in a transistor that switched 5V for future interfacing with the Arduino. This is why the waveform is on the 5V scale.

Scope waveform capture of "Hello, world!"