• Useful

Screen

We have several ways to access and use this system:

  1. Via UART terminal
  2. Via SSH terminal
  3. If Linux has a graphical interface, we can use a keyboard and mouse.
  4. Graphical interface via SSH

UART

Linux on the HPS is usually configured (in the bootloader) to have a terminal redirected to a UART port. This UART port is available via FTDI on one of the board's USBs (the one labeled UART) and can then be accessed as a UART. For this, there is a specific USB port (UART).

Once the USB port is connected to the PC and the board is powered on, we can check which device the serial port was mapped to (on the host):

$ dmesg | tail 
[95158.497894] ftdi_sio 1-1.1:1.0: device disconnected
[95161.649187] usb 1-1.1: new full-speed USB device number 22 using xhci_hcd
[95161.748948] ftdi_sio 1-1.1:1.0: FTDI USB Serial Device converter detected
[95161.749067] usb 1-1.1: Detected FT232RL
[95161.756092] usb 1-1.1: FTDI USB Serial Device converter now attached to ttyUSB0

In the log, we see that an FTDI USB Serial Device was connected to ttyUSB0. We can then open the terminal with any serial port program (PUTTY, ...), in this example we will use the screen program.

$ screen /dev/ttyUSB0 115200,cs8

To improve the interface with screen, create a file ~/.screenrc and add the following content to it:

# Enable mouse scrolling and scroll bar history scrolling
termcapinfo xterm* ti@:te@

⚠️ You may need to install screen.

After this, restart the target (power cycle) to get access to the full boot log.

Note that in my case the name assigned to the USB-UART was /dev/ttyUSB0, this may change on your PC.

Once connected, you now have access to all Linux functionalities as a normal bash. In fact, the terminal is already configured to work at boot, so you can access u-boot information.

Kernel and serial port

How does the kernel know which port to use to print the log and use as a terminal? This parameter is passed by u-boot to the kernel via the Device Tree:

    hps_0_uart0: serial@0xffc02000 {
        compatible = "snps,dw-apb-uart-1.0", "snps,dw-apb-uart";
        reg = < 0xFFC02000 0x00001000 >;
        interrupt-parent = < &hps_0_arm_gic_0 >;
        interrupts = < 0 162 4 >;
        reg-io-width = < 4 >;    
        reg-shift = < 2 >;    
        clock-frequency = < 100000000 >;    
    };

And then passed as an argument to the kernel:

    bootargs = "console=ttyS0,115200";

🚦 (this example is not from our FPGA)