[sdiy] Pots vs Encoders, was Re: [sdiy] dave smith *instruments*

[Scott Gravenhorst]

Jason Tribbeck <Jason.Tribbeck at ascom.com> wrote:
>Hi,
>
>> The PIC will be an I/O controller, handling input from pots, encoders and
>> various switches while also supporting output to the LCD.  There will be a
>> serial interface for connecting the PIC board to the FPGA board, probably
>> SPI.  The FPGA will do all of the synth internals, oscillators, filters or
>> what have you.  I'm using a Xilinx FPGA, so I will use an embedded soft
>> microcontroller (Picoblaze) for MIDI including sysex.  I've already
>> developed the MIDI controller code for other FPGA synth projects and it is
>> tiny.  The advantage I find in doing this is that the Picoblaze controller
>> being right there in the FPGA makes it a matter of HDL to connect it's
>> ports to the synth's registers and RAM.
>
>I did look at the Picoblaze for mine, but I've got some fairly 
>meaty calculations I want to do for the filters (the filters 
>themselves are being done by the FPGA, but the coefficient 
>calculations I want to be done in real-time (well, 100ths of a 
>second). Since I'll have 288 oscillators on the FPGA, I'm not 
>sure if the picoblaze could cope with this. 
>
>Although since it is my first FPGA synth project, I am trying to 
>take it one step at a time! -- Jason Tribbeck 

One of the reasons I want to stay with PicoBlaze for the MIDI controller is that after
development of several FPGA synths, I found that more often than not, I have changed the
MIDI controller code to "fit" a given synth design better.  The "knobby" synth box I'm
designing will likely also be able to support several different synth designs.  If the
MIDI controller is in the FPGA, the structure of the internal parameter RAM can change as
required - including the address space.  Keeping the MIDI controller in the FPGA allows
me to send a single bit file to configure the synth.  Once the I/O controller is bug
free, I should never need to reload it again.

Perhaps it's because I don't have a good "vision" of what I would need in the future - so
I never wrote an all purpose MIDI controller that can be used in any possible synth I
might design.

-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
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