Synth Programmer

[Magnus Danielson]

>   So I spent a couple of hours last night pouring over chapters 10 and 11
>   of The Art of Electronics - trying to absorb as much as possible in the
>   short amount of time. It really doesn't seem that difficult to do uC
>   designs - through it's definitely not the kind of thing I'd want to do on
>   protoboard :).

Pretty good introduction if you haven't done it before...

They even use the 68000 as an example processor there...

>   So, to keep the discussion going, what sort of input/output voltage range
>   should this thing have?
>   0-5v is the obvious choice - this would have a resolution of about
>   .02mV/bit for 8bit conversion.
>   Perhaps some of the values should be stored as 0-5 and some as -5 to +5?

I think that either you just go for doing the full range and spend 1 or 2 extra
bits on that or you use an analogue scale and offset triming for each of the
signals. Doing it the later way allows you to also cope with ranges of -2.5 V 
to
+6 V or something similaily odd. It has the backside that it will defaultly 
cost
extra op-amps and resistors while the upside is that you get full resolution
usage. Maybe just do it as a stuff/non-stuff option but spend the PCB space
rigth away.

>   Anyone want to make a hard and fast vote on a processor? The 68000 sounds
>   very cool and powerful - I also like it's forward-reading assembler
>   notation better than the Intel (bad reason for picking a chip - I know).

Actually, that depends on what assembler you uses, most assemblers do keep the
same format as Intel does, but there is no guarantee...

Besides, for doing just the CV storage things with no extreme things I can do
the assembly by hand, just like in the old days. This way you will not depend
on some software tool that only runs under a distinguished system (no names) :)

>   I'd also be into the 68000 because that's what my uWaver uses, and a
>   uWave has half of the circuitry we're trying to make! It's got one muxed
>   DAC driving 32 control voltages! Not to mention MIDI, an LCD, and a data
>   wheel (heck, it even generates wavetable audio - but who cares about that
>   stuff :). Seems like we could just take the schematics for their digital
>   board, pair them down, add the ADC's, write the firmware - and we're
>   there. We'd need to come up with a choice for an octal sample and hold -
>   because the one they use is CEM and I don't know if it's cheap and/or
>   available...

I thougth we just use standard CMOS 1 to 16 analogue switch chips some caps
and simple op-amp buffers. I think that they would be easy to find for all of 
us.

>   OTOH, half of the group seems inclined towards the 8051 line. If this
>   thing can run a Jupiter I'm sure it can do what we're trying to do. If
>   poeple who can actually do the design are more comfortable with this IC,
>   then that's probably where we should stay. I believe this chip is used in
>   my Fatman - yes? It's probably a cheaper and simpler alternative.

Well, the 68000 line was probably my idea (or fault :) but I have no trouble
with any of them. I think that there is about the same trouble with both CPUs
even if the 68000 may be a little too much for just a CV storage unit.

For this specific project would much out of the 8051 features be unused anyhow.
Maybe only the timer/interrupt would be the extra that isn't really in the
68000. The timer/interrupt would be used to generate timely interrupts to 
preform steady read/updates in a strictly timed fasion.
The 8051 also has some onboard RAM, but since we probably have enougth SRAM 
outside anyway we migth use some of that to do the same work for the 68000
solution.

The 8051 can do MIDI really easy with just initiate a few registers properly
and have the standard CMOS/TTL to MIDI level conversion. An 68000 would need a
external serial chip (like a 6850) do to similar things.

Magnus