1.4.2.8. VGA tutorial

In this tutorial, I will explain how to display a picture on a monitor using Red Pitaya. I used Xilinx Vivado 2021.1 for hardware programming with Xilinx SDK 2021.1 for software applications. The picture is just a simple matrix with “1” and “0” that represent black and white pixels. The picture can be scaled and moved and it can display different patterns.

Required hardware:

• Red pitaya

• Hardware extension for VGA connector

1.4.2.8.1. Step-by-step tutorial

Open Vivado and choose to create a new project, type in the project name and project location. It is recommended to choose a location that has no spaces in the file path because Vivado can have some problems with it. Click next till you come to the page where you have to define what hardware you are using. Select “Boards” and choose Red Pitaya.

Vivado doesn’t have Red Pitaya installed by default so you have to copy board definitions from github to C:/Xilinx/Vivado/…/data/boards/board_files/.

We will use Block design to design our project because it is more manageable, but we will still have to write some VHDL code because not all the IPs we will be using are already implemented in Vivado. We will start by writing code in VHDL and creating our custom-made IPs.

First, we need to define the resolution and frequency of the monitor. Below is a table with values chosen for resolution 800 x 600, with a frequency of 50 MHz.

VGA 800 x 600	Number of periods	Number of rows
period	H = 1040	V = 666
visible section	Hp = 800	Vp = 600
pulse start	Hf = 856	Vf = 637
pulse duration	Hs = 120	Vs = 6

For displaying the picture we need a process that runs line by line on the screen. Below is the process that shifts the cursor on the screen, with a frequency of 50 MHz. (vga_vhdl.vhd)

P1: process (clk50)
begin
    if rising_edge(clk50) then
        clk_div <= not(clk_div);
        if hst_sig < (H-1) then
            hst_sig <= hst_sig +1;

        else
            hst_sig <= (others => '0');
            if vst_sig < V-1 then
                vst_sig <= vst_sig +1;
            else
                vst_sig <= (others => '0');
            end if;
        end if;
    end if;
end process;

A second process is to read the data from the array (picture.vhd).

P2: process (hst_sig, vst_sig, cx_sig, cy_sig)
begin
    if (hst_sig < Hp) and (vst_sig < Vp) then -- and en = '1' then
        if(cx_sig < Hslika) and (cy_sig < Vslika) then
            if slika(to_integer(cy_sig))( to_integer(cx_sig)) = '1' then
                rgb <= "111";
            else
                rgb <= "000";
            end if;
        else
            rgb <= "001";
        end if;
    else
        rgb <= "000";
    end if;
end process;

Image for display

type logo is array(0 to 19) of std_logic_vector(0 to 79);
signal slika: logo := (
"00000000000000000000000000000000000000000000000000000000000000000000000000000000",
"00000000100000000000000000000000000000000000000000000000000000000000000000000000",
"00000001100000000000000000000000000000000000000000000000000000000000000000000000",
"00000001000000000000000000000000000000000000000000000000000000000000000000000000",
"00000001001000000000000000000000000000000000000000000000000000000000000000000000",
"00000001001000000000000000000000000000100000000000000000000000000000000000000000",
"00000001111000000000000000000000000000100000000001001111111111111111111111111111",
"00001001111000000000000000000000000000100000000000001000000000000000000000000000",
"00010011111001000001011001111100011111100011110001011111011111100100000101111110",
"00011111111111000001100010000010100000100100001001001000000000010100000100000001",
"00000000000000000001000010000010100000100100001001001000000000010100000100000001",
"00000000000000000001000010000010100000100100001001001000001111110100000100111111",
"01111110000000000001000011111000100000100100001001001000010000010100000101000001",
"00111110011001100001000010000000100000100100001001001000010000010100000101000001",
"00111100011001100001000010000000100000100100001001001000010000010100000101000001",
"00011110000000000001000010000000100000100100001001001000010000010100000101000001",
"00011111111111000001000001111100011111100111110001000111001111110011111100111111",
"00011111111110000000000000000000000000000100000000000000000000000000000100000000",
"00000000000000000000000000000000000000000100000000000000000000000000000100000000",
"00000000000000000000000000000000000000000000000000000000000000000000000000000000");

Which looks like this:

For the monitor to work correctly, it is necessary to send synchronization pulses at the exact time, for the exact duration (vga_vhdl.vhd).

--signals to synchronize the screen
hsync <= '1' when hst_sig >= Hf and hst_sig < Hf + Hs else '0';
vsync <= '1' when vst_sig >= Vf and vst_sig < Vf + Vs else '0';
rgb_out <= rgb_in;
end Behavioral;

These two code files are packed in a separate IP and have the following simple block diagram.

Before synthesizing the project, do not forget to create a wrapper over the block design (if it is not already created). Otherwise, the top module will not be found

OS version 1.04 or olderOS version 2.00

Please note that you need to change the forward slashes to backward slashes on Windows.

1. Open Terminal or CMD and go to the .bit file location.

cd <Path/to/RedPitaya/repository>/prj/Examples/VGA_image/tmp/VGA_image/VGA_image.runs/impl_1

2. Send the .bit file to the Red Pitaya with the scp command or use WinSCP or a similar tool to perform the operation.

scp system_wrapper.bit root@rp-xxxxxx.local:/root/VGA_image.bit

3. Now establish an SSH communication with your Red Pitaya and check if you have the copy VGA_image.bit in the root directory.

redpitaya> ls

4. Load the VGA_image.bit to xdevcfg with

redpitaya> cat VGA_image.bit > /dev/xdevcfg

If everything is working correctly, you should see a Red Pitaya logo image displayed on your monitor after the cable is connected.

1.4.2.8.2. Automatic generation of the example from the repository

• First, download the Red Pitaya FPGA Git repository to your computer and navigate to the RedPitaya-FPGA/prj/Examples folder.

• Open the make_project.tcl file, uncomment the line “set project_name Vga_image”, and comment all other “set project” lines.

• Open Vivado 2020.1 and in Vivado Tcl Console navigate to the base folder: RedPitaya-FPGA/prj/Examples.

• Then run the script by typing into the following command into the TCL console. If the TCL console is not open got to Tools → Run Tcl Script:

  source make_project.tcl
  

• make_project.tcl automatically generates a complete project in the RedPitaya-FPGA/prj/Examples/Vga_image/ directory.

Take a moment to examine the block design.

If the Block Design is not open, click on Flow => Open Block Design from the top menu or select Open Block Design on the left-hand side of the window (under IP INTEGRATOR). When you are ready, click Generate Bitstream at the bottom-left part of the window to generate a bitstream file.

After you confirm that both Synthesis and Implementation will be executed beforehand, the longer process starts. After successful completion of synthesis, implementation, and bitstream generation, the bit file can be found at Examples/Vga_image/tmp/Vga_image/Vga_image.runs/impl_1/system_wrapper.bit.

Finally, we are ready to program the FPGA with our own bitstream file.

1.4.2.8.3. Conclusion

Congratulations!!! You have successfully created the VGA image project!

If you want to roll back to the official Red Pitaya FPGA program, run the following command:

OS version 1.04 or olderOS version 2.00

redpitaya> cat /opt/redpitaya/fpga/fpga_0.94.bit > /dev/xdevcfg

or simply restart your Red Pitaya.

1.4.2.8.4. Author & Source

Original author: Jaka Koren

Original lesson: link