Re: diy vga signal from ram
From: Rich Grise (richgrise_at_example.net)
Date: 12/28/04
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Date: Tue, 28 Dec 2004 22:55:49 GMT
On Tue, 28 Dec 2004 19:53:16 +0000, Ben Bradley wrote:
> On 28 Dec 2004 12:45:18 GMT, Ian Stirling <root@mauve.demon.co.uk>
> wrote:
>
>>mike <mike@thing.com> wrote:
>>> ok, here i go, perhaps i little clearer this time;
>>>
>>> i have looked at vga timing specs and at max graphics mode res.
>>> (640*480*256), i think the data sourcing (from ram), and subsequent signal
>>> generation suitable for standard vga compliant monitor is possible with
>>> cheap of the shelf components. i don't want to go down the road of pic or
>>> fpga. i don't want to try to implement any hardware acceleration, just
>>
>>How do you intend to fill the RAM?
>
> May I suggest this: have two banks of RAM, one being read out by
> the display circuitry while the other is available to be written and
> read on a microprocessor bus. Have a bit on a port writable from the
> processor that when 0 has bank A switched to the processor bus and
> bank B displayed on video, and when set to 1 has bank B switched to
> the processor bus and bank A displayed on video. You generally don't
> want the processor to write to the active display RAM anyway.
> If you can do the memory updating within one frame refresh time
> (have a polled port available or do an interrupt at the vertical
> retrace so the processor can sync to it), and switch frames at the
> vertical retrace, and you can do "full animation."
>
> But all this will soon be a lost art, as non-CRT displays will
> continue to fall in cost and change to a digital-only connection to
> computers.
>
Do they still make video RAMs, with two address buses? They have sort
of an input side and an output side, and supposedly they can be clocked
independently.
I built a TV typewriter, and I think I based the processor clock on
the video clock and just interleaved bus cycles.
The thing I was most proud of was pipelining the data to/from the
character ROM to give it a couple of cycles to respond at leisure. :-)
You do have to adjust the video timing so the chars still land on the
screen. ;-)
Cheers!
Rich
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