Bitmap VGA output

WARNING : The module has recenty been updated… some information here might be obsolete… contact me for confirmation.

Before the implementation of any of these modules, please, check if your design leaves enough memory resources for the required memory. Logic requirements are negligible and working frequency is high enough in most cases.

( figure source )

• n : number of address bits (resolution dependant)
• bpp : number of bits/pixel (configurable)
• reset is active high

The most simple version is to be instanced with the following lines. Optional I/Os are not used, and might produce warnings, you can safely ignore them.

  display_module : entity work.vga_bitmap_640x480
      generic map(RAM_BPP  => 4,            -- number of bits per pixels
                  INDEXED  => 0,            -- do not used indexed colors
                  READBACK => 0)            -- read from bitmap memory disabled
                  
      port map(clk          => clk,         -- 100MHz system clock      
               reset        => reset,       -- active high system reset
               
               VGA_hs       => VGA_hs,      -- VGA screen output
               VGA_vs       => VGA_vs,
               VGA_color    => VGA_color,
               
               pixel_x      => pixel_x,     -- pixel horizontal coordinate
               pixel_y      => pixel_y,     -- pixel vertical coordinate
               data_in      => data_in,     -- new color for the addressed pixel
               data_write   => data_write); -- write order

If data readback is required, it becomes :

• warning: a 5 to 6 clock periods latency is necessary between the data_read assertion and data_rout assertion
• data read requests can be pipelined

  display_module : entity work.vga_bitmap_640x480
      generic map(RAM_BPP  => 4,            -- number of bits per pixels
                  INDEXED  => 0,            -- do not used indexed colors
                  READBACK => 1)            -- read from bitmap memory enabled
                  
      port map(clk          => clk,         -- 100MHz system clock      
               reset        => reset,       -- active high system reset
               
               VGA_hs       => VGA_hs,      -- VGA screen output
               VGA_vs       => VGA_vs,
               VGA_color    => VGA_color,
               
               pixel_x      => pixel_x,     -- pixel horizontal coordinate
               pixel_y      => pixel_y,     -- pixel vertical coordinate
               data_in      => data_in,     -- new color for the addressed pixel
               data_write   => data_write,  -- write order
               
               data_read    => data_read,   -- read order
               data_rout    => data_rout,   -- data read is ready
               data_out     => data_out):   -- pixel read color

The end_of_frame output signal might also be useful to synchronize display updates with screen refresh.

Pixels are addressed by x,y coordinates from the top left (0,0) to the bottom right (xmax, ymax).

Different resolutions are available :

• 160×120 (on request)
• 320×240
• 640×480

All resolutions are available from 1 bit/pixel to 12 bit/pixel. It is possible to chose between colored or greyscale for color depth of 2, 3 and 4 bits / pixel.

The graphic memory only contains 1 bit/address to match the pixel coding. The output is then black for '0' and white for '1'. As the VGA interface is coded using 4 bits / color (or 12 bits / pixel). It is possible to modify the white value by any other with simple logic gating.

The pixel value is encoded with 2 bits. The generic parameter grayscale makes it possible to code the screen as monochrome if set to true, if omitted, it is considered as false and display will be colored. The corresponding values are as follow :

In each case, the color is set at its maximal intensity.

Here again, the generic parameter grayscale makes it possible to code the screen as monochrome if set to true, if omitted, it is considered as false and display will be colored. With this coding scheme, each bit represent a primary color. Each individual color is always set at its maximum. The next table lists the expected colors.

The 4-bit representation is very similar to 3-bit. There is the possibility to use it to code grayscale or colored pixels. For colors, it uses the same scheme as 3-bit for the chrominance, and uses the MSB (4th) bit to code luminance. When the MSB is 0, colors are dark, when the MSB is '1', colors are bright. It is worth mentioning that dark white is a gray which is lighter than bright black.

Using this color scheme, colors are coded in RGB format where R, G and B may take three values (dark or 0, medium or 1, bright or 2). The corresponding values are then coded as an integer given by : (R*9 + G*3 + B). This representation only provides 27 different colors where 32 are actually possible, but it is a good compromise to provide RGB coding with low memory resources.

Beyond this point, pixel values are coded using RGB representation. The table below shows how bits are split to provide the expected coding :

• file for 320x240 pixels resolution
• file for 640x480 pixels resolution