RE: ISA Vector card already done...

> And the serial update is fast enough? I remember looking at serial
> D/A and for
> a 6mhz update needed for an asteroids speed display (all from memory
> -- maybe it
> was 3mhz?) for a dual channel 12 bit port you needed a 144mhz data
> clock going
> into the thing. Does the AD7249 really operate that fast? Or am I
> missing
> something -- which could easily be the case...
>
Hmmmm, good point. The serial update rate is 2MHz, but that only gives
you a 125KHz DAC update rate. The ADSP-2181 looks like it'll probably
max out at about 1.25MHz for serial update though (so about 600KHz for
Dual DACs...).

Huh. Might have to go with parallel DACs. :-/ No big deal, serial is
cool on the DSP 'cause it hardware updates and takes no clock cycles to
load the DACs. But, there will probably be some spare DSP MIPs left
anyway, so memory-mapping some DACs shouldn't hurt. Must fast DACs will
probably work with 0-wait states anyway.

How did you calculate the speed required for the DACs for your Asteroids
example?

Off the top of my head I'd say:

        Resolution of "screen" = 1024x1024 (10 bits each direction
significant)
        Max vectors = 2000 (decided by wild-ass-guess)
        Average Vector length = 100 clocks (about 1.5 inches on a 19"
screen-- seems generous)
        Update Speed = 60Hz (16.67ms per frame)
so...
        2000 * 100 = 200,000 clocks total
        16.67ms / 200,000 = 83ns per update ...
        ~12Mhz?

So that's about 2.5 DSP instructions per pixel, which if you need to
update two DACs is probably pretty tight. I could go to the 40MIP part,
or we can say that the above is "worst case" for 60Hz, and if you push
it harder you'll just drop frame rate a little...

Whatcha think?

-Clay
Received on Wed Mar 25 11:00:00 1998