A VGA display interface needs five signals to display a picture:
1) The R, G and B signals are red, green and blue.
2) HS and vs (horizontal synchronization signal and vertical synchronization signal).
R. G and B are analog signals, HS and vs are digital signals.
Here is how to drive the VGA connector:
The pins 13 and 14 (HS and vs) of VGA interface are digital signals, so they can be controlled directly through the two IO ports of FPGA (or they can also be connected with low resistance, such as 10 or 20 Ohm).
Pins 1, 2 and 3 (R, G and b) are 75 ohm analog signals with a constant value of 0.7V. Output by 3.3V FPGA pin, with three 270 ohm resistors just meet the requirements. The input voltage driving resistance of the connector is 75 Ω, so the 3.3V becomes: 3.3 * 75 / (270 + 75) = 0.72v, very close to 0.7V.
The 5, 6, 7, 8 and 10 pins are grounded.
The scan mode of VGA is non cross scan.
Synchronous polarity: both horizontal and vertical directions are low level active.
Definition of VGA interface
The VGA interface corresponding to the pin is as follows:
FPGA directly uses IO port to connect five VGA signal interfaces, and the input of three primary color signal interface can only be digital signal (0 or 1), so there are only eight kinds of colors displayed on LCD screen. Generally speaking, a Da module can be added between FPGA and VGA interface, and the color matching effect can be determined according to the size of Da bits. In theory, each signal can have 256 level values (that is, dividing the 0-0.7v voltage into 256 equal parts), so the three RGB signal interfaces will have 256 at most, which is more than 14 million colors. In the design of true color, ad module is generally needed. But if you only use VGA to display some simple industrial menus, you don’t need so many colors. Maybe 8 colors, maybe 64 colors or 256 colors are enough.
Using the resistance network in the figure above to achieve Da effect, VGA can get 4x4x4 = 64 colors. This is enough in industrial control.