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diy vga signal from ram

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mike

Jan 1, 1970
0
i'm looking for info re. producing vga signal based on contents of ram.
don't want to use pic or fpga, from what i've found so far it looks do-able
with the right clock freq. , some sram and a fistfull of 74XXXXX ic's.
can anyone point me in the direction of some web based gen.?

thanks for any info received..

mike
 
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Rich Grise

Jan 1, 1970
0
i'm looking for info re. producing vga signal based on contents of ram.
don't want to use pic or fpga, from what i've found so far it looks do-able
with the right clock freq. , some sram and a fistfull of 74XXXXX ic's.

More like a bushel basket full. )-;

Good Luck!
Rich
 
L

Leon Heller

Jan 1, 1970
0
mike said:
i'm looking for info re. producing vga signal based on contents of ram.
don't want to use pic or fpga, from what i've found so far it looks
do-able
with the right clock freq. , some sram and a fistfull of 74XXXXX ic's.
can anyone point me in the direction of some web based gen.?

Why bother when you do it with a small FPGA or CPLD, an oscillator module,
three resistors and three diodes.

Leon
 
D

Dave Garnett

Jan 1, 1970
0
see The Cheap Video Cookbook, by Don Lancaster

Dave
 
D

Don Taylor

Jan 1, 1970
0
mike said:
i'm looking for info re. producing vga signal based on contents of ram.
don't want to use pic or fpga, from what i've found so far it looks do-able
with the right clock freq. , some sram and a fistfull of 74XXXXX ic's.
can anyone point me in the direction of some web based gen.?
thanks for any info received..

I'm groping for the info and I can't put my hands on it but...
I think I remember something in Circuit Cellar in the last year
where someone did this with a surprisingly small number of parts
and fitting into your constraints. I seem to remember that he
saw he could do this in one mode with a couple of counters and
a little glue logic, substantially less than a handful of 74XXXXX.
 
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Ian Stirling

Jan 1, 1970
0
Don Taylor said:
I'm groping for the info and I can't put my hands on it but...
I think I remember something in Circuit Cellar in the last year
where someone did this with a surprisingly small number of parts
and fitting into your constraints. I seem to remember that he
saw he could do this in one mode with a couple of counters and
a little glue logic, substantially less than a handful of 74XXXXX.

It's not hard.
Especially if you can cheat and use a bit or two in the RAM as state controls.
I did a simple design for a Z80 based computer with a graphics output.

Basically, a couple of gates to make a counter read a byte from an address
in SRAM when the Z80 wasn't using it, and a shift register to read it out a
couple of bits at a time and present it to a DAC.

The Z80 fills in the sync pulses (DAC has 3 levels and sync) and does all
drawing.

IIRC, there were about 8 chips, and a few resistor arrays, counters,
SRAM, shift register, ...
(No eprom, it loaded in the program using the shift register and counter, used
the wrong way round)
 
M

Mike Monett

Jan 1, 1970
0
Ian Stirling wrote:

[...]
It's not hard.
Especially if you can cheat and use a bit or two in the RAM as state controls.
I did a simple design for a Z80 based computer with a graphics output.

Basically, a couple of gates to make a counter read a byte from an address
in SRAM when the Z80 wasn't using it, and a shift register to read it out a
couple of bits at a time and present it to a DAC.

The Z80 fills in the sync pulses (DAC has 3 levels and sync) and does all
drawing.

IIRC, there were about 8 chips, and a few resistor arrays, counters,
SRAM, shift register, ...
(No eprom, it loaded in the program using the shift register and counter, used
the wrong way round)

This sounds interesting. Can give some more info such as links, schematics, and code?
Any articles?

Isn't there three dacs, one for each color? Does this mean you can have three
independant AWG's? What resolution can you get? What is the sample rate? How do you do
low frequencies?

Someone mentioned the blanking interval might mess up the waveform - how do you get
around that?

Best,

Mike Monett
 
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Ian Stirling

Jan 1, 1970
0
Mike Monett said:
Ian Stirling wrote:

[...]
It's not hard.
Especially if you can cheat and use a bit or two in the RAM as state controls.
I did a simple design for a Z80 based computer with a graphics output.

Basically, a couple of gates to make a counter read a byte from an address
in SRAM when the Z80 wasn't using it, and a shift register to read it out a
couple of bits at a time and present it to a DAC.

The Z80 fills in the sync pulses (DAC has 3 levels and sync) and does all
drawing.

IIRC, there were about 8 chips, and a few resistor arrays, counters,
SRAM, shift register, ...
(No eprom, it loaded in the program using the shift register and counter, used
the wrong way round)

This sounds interesting. Can give some more info such as links, schematics, and code?

Nope.
Somewhere I've got a schematic in pencil, and a big timing diagram.
Any articles?

Isn't there three dacs, one for each color? Does this mean you can have three
independant AWG's? What resolution can you get? What is the sample rate? How do you do
low frequencies?

2 bit, black, grey, white and sync.
It was designed to do PAL TV output.
Someone mentioned the blanking interval might mess up the waveform - how do you get
around that?

Simply set areas of the display to 'sync' level.
The counter doesn't know about lines, interlacing, ...
It's all done in software.
In the above case, I was able to not even bother about overflow on the
counter, as the clock rate was set so that it read out the entire frame
memory in one frame time.

Nowadays, you'd almost have to be insane not to do it with programmable
logic of some sort.
Especially for VGA and colour.
The bitrate and required frequencies are high for TTL.
 
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Mike Monett

Jan 1, 1970
0
Ian Stirling wrote:

[...]
Nowadays, you'd almost have to be insane not to do it with programmable
logic of some sort.
Especially for VGA and colour.
The bitrate and required frequencies are high for TTL.

OK Ian, thanks for the info. I was thinking this might be useful if you
were stuck on a desert island one Saturday night and needed a quick hack.
Sounds like it might take a while to get working, and may have some
limitations that would be difficult to work aound.

I guess I'd better stock up on Analog Devices DACS:)

Best,

Mike Monett
 
M

mike

Jan 1, 1970
0
Don Taylor said:
I'm groping for the info and I can't put my hands on it but...
I think I remember something in Circuit Cellar in the last year
where someone did this with a surprisingly small number of parts
and fitting into your constraints. I seem to remember that he
saw he could do this in one mode with a couple of counters and
a little glue logic, substantially less than a handful of 74XXXXX.

thanks for the lead and the encouraging attitude.

mike
 
M

mike

Jan 1, 1970
0
Ian Stirling said:
Nowadays, you'd almost have to be insane not to do it with programmable
logic of some sort.
Especially for VGA and colour.
The bitrate and required frequencies are high for TTL.

that's all right then, i am almost insane...

mike
 
M

mike

Jan 1, 1970
0
Don Taylor said:
I'm groping for the info and I can't put my hands on it but...
I think I remember something in Circuit Cellar in the last year
where someone did this with a surprisingly small number of parts
and fitting into your constraints. I seem to remember that he
saw he could do this in one mode with a couple of counters and
a little glue logic, substantially less than a handful of 74XXXXX.

i found the article, by enoch hwang just this november, just paid for the
pdf on the site (only $1.50) but can't get the password (page unavailable
!!). have emailed the webmaster, not holding breath due to time of year...

thanks again for the tip

mike
 
R

Rich Grise

Jan 1, 1970
0
Ian Stirling wrote:

[...]
Nowadays, you'd almost have to be insane not to do it with programmable
logic of some sort.
Especially for VGA and colour.
The bitrate and required frequencies are high for TTL.

OK Ian, thanks for the info. I was thinking this might be useful if you
were stuck on a desert island one Saturday night and needed a quick hack.
Sounds like it might take a while to get working, and may have some
limitations that would be difficult to work aound.

I guess I'd better stock up on Analog Devices DACS:)

Wouldn't it almost be "easier", in this day and age, to just build
a(an?) 8x8x8x2048x1600 video thing and not screw around with VGA specs?
As long as you get the sync freqs right, of course. :)

If you already have something making VGA and just want to display
it, that's just an RGB monitor, and if you're trying to build one
of those, you _are_ insane. ;-)

And then you have the issue of getting the data in and out. Well,
in.

Good Luck!
Rich
 
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Rich Grise

Jan 1, 1970
0
thanks for the lead, i'll look into it.
A TV Typewriter, you can build with a few chips. Even color! ;-)

A VGA, on the other hand, is not a hobbyist project.

Have Fun!
Rich
 
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Ian Stirling

Jan 1, 1970
0
mike said:
that's all right then, i am almost insane...

Well, I have occasionally wondered about getting LEGO to sponsor me to
make a mechanical CPU...
 
M

mike

Jan 1, 1970
0
Rich Grise said:
A TV Typewriter, you can build with a few chips. Even color! ;-)

A VGA, on the other hand, is not a hobbyist project.

Have Fun!
Rich

with all due respect;

your implied suggestion that certain types of project are unsuitable for non
proffessionals (is that what is meant by 'hobbyist'?) isn't really very
helpfull (or friendly). i really don't want to start one of those pointless
flame-war slanging matches with a negative comment, but i do feel strongly
that all interested efforts, however flawed or over ambitious, should be
encouraged at the very least. if i have a go and fail miserably, don't
worry, i'm a big boy. i'll probably have a go at making a "space elevator"
out of fishing line or something similar.like you say, for the "fun". just
don't tell me i can't.

thanks for reading, if not contributing.

mike
 
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Rich Grise

Jan 1, 1970
0
Rich Grise said:
On Fri, 24 Dec 2004 23:18:40 +0000, mike wrote:
[quoting Rich Grise]
A TV Typewriter, you can build with a few chips. Even color! ;-)

A VGA, on the other hand, is not a hobbyist project.

with all due respect;

your implied suggestion that certain types of project are unsuitable for non
proffessionals (is that what is meant by 'hobbyist'?) isn't really very
helpfull (or friendly).

No, it's not very helpful at all. But do you want to delve into the
morass of VGA and SVGA specs to dope it out? They have very large
logic array things on those things.
i really don't want to start one of those pointless
flame-war slanging matches with a negative comment,

Neither do I.
but i do feel strongly
that all interested efforts, however flawed or over ambitious, should be
encouraged at the very least.

Sure! Go track down the VGA spec! No big deal at all! Then, tell me
if you want to build one from scratch with TTL.
if i have a go and fail miserably, don't
worry, i'm a big boy. i'll probably have a go at making a "space elevator"
out of fishing line or something similar.like you say, for the "fun". just
don't tell me i can't.

I never said any such thing.

You can do anything your want to do.

By "hobbyist," I meant the kind of guy who's still learning to put
current-limit resistors on LEDs, and that sort of thing. I've done a TV
typewriter at a "Hobbyist" level. Calling oneself a "Hobbyist," but
claiming to take on a full-on VGA from scratch is a stretch.
thanks for reading, if not contributing.

Well, you are certainly more then welcome to look into "How to make a
VGA from scratch using TTL", but I can't guarantee that you'll have
much success, unless you bite off little chunks, like a 640 X 480
graphics TV Typewriter with VGA sync speeds.

THEN you start on the graphics acceleration and all that crap. OK?

Remember, you'll need three DACs that can spit out analog video at
better than 6 MHz, probably more like 15 MHz.

If you feel like you're ready to take on that challenge, then by all
means, more power to you!

I just wouldn't necessarily call it a "Hobbyist" project.

Thanks,
Rich
 
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