VGA mode questions

rekarp

I am working on a dual propeller setup. The master propeller does most of the grunt work and the like and the slave propeller will be doing only video and image display in either VGA or composite (haven't decided yet). The master communicates by sending the data serially in 32 bits. It uses 32 as the master already has a CLK and LATCH for controlling 2 sets of 4 cascaded 74HC595's and 2 sets of 4 cascaded 74HC165's and to save I/O I can use the same ones. Theres also a set of 20 cascaded 74HC595's that control a display but that uses a different CLK and LATCH.

Now after receiving the 32 bits the slave just goes to a lookup table it has and sees what it needs to pull from its SD card. Now what I want to know is after the FAT driver and the little bit of code to do the connection between the two props is what would be the best image I can pull out of the slave propeller? I have the connection working just need to know where to procede to get the best image. If I use NTSC I am hoping for atleast 128 colors.

Thanks

Mike Green

The Propeller video hardware can produce 4 colors per pixel chosen from a palette of 4 x 256 color values. Each group of 16 pixels on a horizontal line can use a different set of 4 colors. It's possible to "fool" the hardware to handle more colors, but it's not for the beginner. Also keep in mind that there is only 32K of RAM to use for both code and data including the screen buffer. A 256 x 128 pixel screen is 32K pixels. At 2 bits per pixel, that's 8K bytes which is 1/4 of available memory.

Paul Baker

I believe you will run into difficulty getting 128 colors in NTSC from the Propeller, this is because the chroma information is phase encoded and there are a limited number of phases the propeller can produce. There have been attempts to extend this by giving two different colors during a pixel window, but this technique works best with colors near each other in color space.

It is easier to obtain a higher number of colors with VGA, but you start to run into the issue of sacrificing resolution for additional color.

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Paul Baker

Tubular

Hi Rekarp

I've been wrangling a similar dual prop design this week which has a slave "media" prop. I've decided to go with VGA as slim connectors are available that don't eat much board space, and I'm trying to keep it similar to a DIP40 footprint.

The media slave has its own eeprom and uSD card, so like you I'm hoping to keep as much of that 32k for screen buffer as possible, most of the time, but to be able to "page" in code from the eeprom or uSD card as required.

cheers
tubular