I'm thinking about making a project like this one and add a graphics card like that one. Using an EEPROM for the video memory and changing its content setting it in write mode and assigning a value for every pixel that need to be change. The problem is that the CPU will run at a very slow frequency, so the change will require some time and in this interval the monitor will show nothing (if I will disconnect the output signal during writing) or constant color lines depending of the value that is being changed (if I will leave the output connected).
Is there a way to "freeze" the image on the monitor during this operation? Or are there better solutions?
Till now my last idea was to use two EEPROM with the same content and change the value in one memory while reading the data from the other, then read from the first memory while writing the same data in the second one (flipping the roles of the two EEPROM); but it seem extremely inefficient to me.
PS: I didn't find any documentation for freezing the image using simply the VGA lines.
- I was thinking to use ROM so that I can preload a fixed background and change fewer pixels while running (to save time and memory)
- This will be just for fun, without any real practical use, so I didn't think a lot about the frequency, but I can imagine from few Hz (while debugging or for demo purpose) to few kHz, while the "video clock" will probably be 10 MHz, so write in precise, small and synchronized intervals is not a easy task, I think.