RE: Cinematronics sound board behavior...

> Remember the CCPU runs at 5mhz with many of it's
> instructions being executed in 1 cycle. You need to be very fast to
> catch the shift strobe of a shift register, which I believe will only
> last for two 5mhz cycles!
>
I was kinda thinking 16V8 or something to just latch the info... The Scenix
might be able to do it, but for a dollar or two it might just be worth the
PAL...

It looks like the Scenix at 50MHz can outrun the SmartMedia card. :-) I
think the numbers work out for CPU power.

I can set the address on the memory and retrieve a byte of data in roughly
29 Scenix cycles (20ns each). That doesn't include any overhead obtaining
the address from Scenix RAM etc. I can then grab subsequent bytes from the
memory in just 5 scenix cycles.

So, If I (arbitrarily) set 22.1KHz as the playback rate, I've got about
45.249us before I need to pump out a word from the DAC. Sending a word to
the DAC will take about 50 scenix cycles. Soooo. Let's just take a
wild-ass guess and say there's 100 cycles of math and overhead needed to
combine samples and do "whatever" (poll inputs, etc).

Overhead + output = ~150 cycles = ~3us
Getting one "voice" of data = ~30 cycles = ~600ns

Needing a new word written to the DAC takes 3us from our 45.249us refresh
rate. That leaves about 42.25us for fetching data from the SmartMedia card
or at 600ns per voice... about 70 voices without per-voice overhead.

The conclusion here looks to be that even generating eight or so "virtual"
sound channels leaves lots of CPU left over. Even if I totally botched the
timing diagrams and I'm off by a factor of 5 there's still enough "oomph" to
do the job. ;-)

> Another IRQ task could supply the data to the DAC as needed, with a
> foreground task doing the mixing/level shifting, keeping a very small
> (4 to 8 byte) buffer full for the DAC.
>
Yeah, there's about ~130 bytes of scratch RAM to use. Taking advantage of
the sequential reads on the SmartMedia card to buffer (8?) bytes per voice
per access frees up even more of the lots-o-free MIPS. Hmmmm. I've got a
bunch of 20MHz oscillators I bought... Maybe just use those to be cheap...

> My biggest concern is that the different soundboards interface quite
> different from each other. There will have to be a different routine
> written for nearly each card, I'm wondering if the Scenix has enough
> code space for this.
>
Yeah, we've got about 2K words of codespace. Worst case we could have a
little PIC 16C54 out there to just be the IO procesor. Couple bucks...

> Another idea may be to have an external processor (with a lot of code
> space and the ability to run "Real C" code) as the overall controller.
> Reading information from the Scenix (acting as the I/O interface to
> the CCPU), doing the waveform playing logic, the mixing, etc. and
> sending the results back to the scenix which could buffer and play the
> sounds out the DAC.
>
Ehhh... I forgot just how much speed the Scenix really has. I don't think
we'd need any assist except to maybe just make things easier. The Scenix at
50MHz spends four cycles bit-banging the 80ns /WE clock on the SmartMedia
card... The AD5310 (a single supply, serial, voltage out, 10bit DAC in a
neato-little6-lead SOT 23 package) only needs about 33ns per cycle (the
Scenix would actually do about 60ns bit-banging it) so it's plenty fast.

> But however it's done, I'm willing to work out all the interface logic
> for all the sound boards.
>
That would be really cool. What's the scoop on the Cinematronic Multigame
anyway? Haven't heard anything in a while...

-Clay
Received on Wed Jun 23 20:26:02 1999