[Coco] TC-9

[Chuck Youse]

On Mon, 2008-07-07 at 18:02 -0500, Mark Marlette wrote:
> Chuck,
> 
> There are free CPU cores, etc. Some have dabbled here but there is no  
> GIME FPGA, others will say it isn't that hard, etc. Proof is in the  
> pudding. If it isn't that hard, then do it, no time, busy, etc...Not  
> implying you here.....There is a lot going on in the GIME.
> 

Well, one of the problems with implementing the GIME as a stand-alone
FPGA stems from the fact that the GIME performs quite a few analog
functions, and the digital functions on any suitable FPGA will be at the
wrong voltage levels.  E.g., composite and RGB outputs, not to mention
the clock generation - as I'm sure you know the 6809 likes to drive its
internal circuitry off the E clock.  So any drop-in GIME replacement
would have to be a board of sorts, an FPGA with some external
components.  

If it's a matter of implementing a 6809 + GIME on a chip and interfacing
that to an external bus, that's one thing.  If it's a drop-in board to
replace the GIME, that's another.  But as I stated before, if you're
going to go that far, might as well integrate a lot more into the FPGA.

If your comments were meant as a gentlemanly challenge, I'll take it!
But I want a free AT keyboard interface for my CoCo when I finish ;)

> Some have booted to a green screen, etc but we all know that there is  
> a lot of work in the details of a project and to get one that works,  
> 95%+ of the apps requires a bunch of time. Details, details, details.
> 

If I were to do this, I'd probably put a 6809 and a GIME-style
MMU/interrupt controller on a chip, and interface that to a multi-slot
Coco-style hardware bus through some LVX-family transceivers or
something.  The core+RAM can run at one speed, and external bus accesses
can be synchronized to a free-running E clock @ 2MHz, much like the
early 68ks did for 6800 peripherals.

I wouldn't bother with the graphics functions of the GIME, as I don't
care for RSDOS compatibility - might put in some logic to drive a VGA
display and keyboard, but certainly not in a CoCo/SAM compatible
fashion.

> Then the cost. Last I checked some of the FPGA prices were $250+. Sure  
> there are bigger, better and faster ones now but my point is the same.  
> This is a labor of love and small volume manufacturing costs more than  
> one would think unless they have lived it.

A suitable FPGA with more than enough area for a 6809 and GIME (with
room to spare) would be an XC3S250E - the 144TQFP/4ns speed grade runs ~
$15 at qty 1. 

> I tip my hat to you and your FDC controller project!!!! Nice job.

Thanks!  Means much coming from Cloud-9. :)

C.