On Sat, Aug 26, 2006 at 02:53:57AM +0100, Peter TB Brett wrote:
> Having said that, chaining onto the end of the SPI bus is the most reasonable
> solution I've heard so far. I'd suggest a PIC + serial port as the tidiest
> solution, requiring the following components:
That's one of the higher-capability tools you could hang on the SPI
bus. I suggested a 68HC908AB32, but there are PICs that would work too.
Depending on the raw hooks in the ASIC, you could use that to set the
boot-time video mode, simulate an off-board EEPROM that substitutes itself
for the on-board EEPROM, write a real off-board EEPROM, write to the
on-board EEPROM, or if the on-chip logic to support it is affordable,
provide an TRV10 debugger interface through its RS-232 port.
An SPI tool that sets the video mode on power-up and does nothing
else could be a lot simpler. All it needs is about 120 DIP switches and a
shift register chain to read them in. Its advantage is that it needs no
on-tool code. Zero. That means you don't need some other computer to load
its program or write its mode data into non-volatile storage before you can
set it and plug it in. You just set the switches and power up. That
abolishes all chicken-and-egg paradoxes.
If it's left in the computer to support a fixed-frequency monitor,
it could probably be mounted through standoffs on holes in the video board.
(Or it could be mounted to the chassis on sticky-back Velcro, like a clock
battery. That's a detail.)
> Quick question for users of fixed-frequency monitors: how do you change your
> BIOS settings, seeing as the BIOS of every PC-compatible I've ever seen uses
> VGA? I'm curious... I'm kind of assuming that you're not using PCs!
You need a fixed-frequency video board. Like the one I'm using
right now, which I got from a company that specializes in fixed-frequency
video boards. They find video chips that can be made to work that way,
build a board around them, and provide switch-selectable banks of preset
modes for various common fixed-frequency monitors. If you have an unusual
fixed-frequency monitor, you have to send them the specs and have them
specially configure the board.
Once you have that, you can get a console display, and then you can
enter whatever BIOS settings you need to bring up the rest of the computer.
But you have to get a usable text-mode display first.
Incidentally, I'm repeating stuff we've gone over repeatedly on the
list before. No more than two seeks ago we went over the cost of a
DIP switch board.
Tim and I have had an off-list thread during the last week, because
I didn't think it was necessary to take up everybody's time on small
implementation details. Maybe I was wrong.
The question was, what is the least amount of extra logic the ASIC
would need, to provide a hook for some external tool to set a completely
arbitrary video mode at boot time? The answer seems to be, very little, if
you make use of the SPI port which is already brought out to the header.
And once you do that, it doesn't take much more to make the SPI port provide
other capabilities. One of the most valuable add-on features is probably to
let it detect an off-board EEPROM, so that experimental code loads can be
tested without disturbing the factory EEPROM load. That allows users and
system developers to experiment with their boards, and not risk creating a
doorstop or taking up Traversal's time recovering from errors.
Now let me talk a little about philosophy and human motivation.
One of the things that makes open source projects great is that
there no marketers and managers standing in the way, telling people who want
or need a feature that they can't have it. "Scratch your own itch" is one
of the fundamentals of open source software projects. Here, we're trying to
bring some of those advantages to hardware.
Well, the thing that brought me to this project is the desire for a
future-proof video board with a fixed-frequency capability. That's why I
contribute to this project. I don't try to tell all the people who want hot
3D capabilities that they shouldn't be spending time on that, just because I
don't care about it. If that's what brings them to this project, I respect
their objectives and don't argue against them. This way, we get the
participation that will eventually bring us a fully documented graphics
chip, and boards built around it.
I think the phrase Tim used last time was the "long tail" of the
demand curve. If serving it is cheap enough, then it's cost-effective.
And that's as much ranting as I intend to do.
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