In SX Microcontrollers, SX/B Compiler and SX-Key Tool, cbmeeks wrote:

Some things I have discovered in my VGA circuit design (vs my NTSC color circuit) are a lot more challenging that I thought.

For example, in NTSC, you basically get 160 pixels and running 78.74999 Mhz, you can get 22 clocks per pixel.

In VGA, 640 pixels is pretty much hopeless even at 80 Mhz.  256H pixels (my target res) can give you about 8 clocks per pixel at 80Mhz.  In reality, that 8 clocks is reduced to 4 because a JMP takes 4.  (then again, I haven't researched quicker loop methods yet).  So that means 4 clocks per pixel.  An IREAD takes 4!  Now using 15ns SRAM (and each SX clock taking 12.5ns) you pretty much need 2 clocks to increment an address bus.  Bottom line, MUCH less bandwidth per pixel in VGA.


A 160x192 pixel NTSC image (16 color) takes 15360 bytes.  A 256x192 pixel VGA takes 49152 bytes.  Granted, that's using 64 colors...16 colors would take 24576 bytes.  So more memory for an image.


NTSC has that "coolness" factor....VGA does not (well, not to many).

Anyway, I guess what I am getting at is that to me, these challenges are what is making my VGA circuit fun.  I mean, why even bother with VGA unless you are going to meet what I consider minimum specs for a mcu.  (256x192 with 64 colors).

Of course all of these is based on not using any external video parts (RAMDACs, etc).

You thoughts?  :-)

cbmeeks

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