Spartan 3e starter kit & Multimedia

Not all that difficult to add on another VGA interface with more bits. One of these could come in handy:

Red.

It would, however, have used up more IO pins. I don't know if that was a consideration, but they do seem to share pins for the devices on the SPI bus.

Does anyone know if doing PWM on the VGA output pins would cause Bad Things to happen to a typical monitor?

It would have to be above the video clock rate, and then it would depend on the slew symmetry of the video amplifier.

For a LCD monitor, it would be more unpredictable, as they sample the analog IP to create the digital drive signals.

-jg

: > I just received my Spartan 3e starter kit and had to share my : > frustration... : >

: Does anyone know if doing PWM on the VGA output pins would cause Bad : Things to happen to a typical monitor?

I tried it with the Spartan 3 kit from Xilinx/Digilent a while back and had some success with PWM - if you count success to mean working on 1 of

cds

The problem with PWM here is that there is no filter on the output, so you get essentially an AC signal that will look pretty rough to a monitor expecting a DC signal. To get a true DC value out of that, you need some capacitance. It is possible that if you created a small adapter with three low-pass R-C filters for each signal, you might get acceptable results - even on an LCD.

Alternately, you might be able to solder some capacitors directly to the connector and solder the negative terminals to ground. This depends on the resistance of the board traces - which may or may not be adequate.

There are resistors (few hundred ohms, different for the rgb than for the syncs) between the FPGA pins and the VGA connector.

But two voices of experience have already chimed in saying that straight pwm without board mods doesn't work too well. And if you are going to do something like solder up capacitors, you may as well just make your own VGA port with as many bits as you want.

Perhaps - but three SMT capacitors, a bit of wire, and perhaps a bit of glue is a lot cheaper than rolling your own PWB. This would be a very simple board mod that almost any beginner could successfully manage, and it would be very effective. Cripes - I'm no expert at soldering, but even I've managed to modify boards in this way.

Also, consider that this allows you to keep all your GPIO (which is rather limited on the S3E starter kit). If you are successful, you can still use the hirose connector for something else (like a video capture board)

Lastly, not everyone can afford to roll their own PWB's with a true VGA DAC and connector. My wife would beat me silly if I even contemplated such a thing. So, if I had the board myself, it would either be roll the filters on the board, or live with 3-bit color.

In some ways this is very similar to dithering. I already used ordered dithering to improve the color resolution from 8 to 14bit on another board. Unfortunately 3bit color resolution is extremely low to start with.

One way out could be to use a PAL/NTSC encoder. Is there any free core out there?

If you are willing to use up the Hirose port, you could probably kludge up some R/2R ADCs with the resistors hanging out into free space off the pins of a through-hole connector, with no component more than one component away from a connector pin. Not exactly robust, or sound engineering practice, but usable enough. And not much worse than what I was thinking of, which was a clump of capacitors plugged into the board's VGA socket and leading to a VGA socket to plug your monitor into.

But it does use up most of your expansion capability. Does anyone make a prototyping board with the Hirose connector? Digilent has some for the pair of 2x20 pin headers on their boards, but not for this new board.

But the SMT capacitors solidly attached to the board probably would be a better way to go.

Another idea, and I don't know if this is possible. When you set up the constraint file, you indicate what strength each pin gets driven at, and what logic family it simulates. So could you have e.g. a 1.5 V logic at 2 mA for dim and 3.3 V driving 15 mA for bright? Or are the bits that control that not accessible to the logic signals? Or is it otherwise a Bad Idea?