[Coco] Re: [Color Computer] 1M simms in Cloud9 SIMM upgrade kit

[j_e_daggett]

--- In ColorComputer at yahoogroups.com, Mike Pepe <lamune at d...> wrote:
 
> thanks again everyone... I guess until we make a FPGA-GIME replacement 
> we're stuck.

******

Well yes and no! An FPGA part can be made to be somewhat backward
compatible. You could put the FPGA on a board and have fingers that
plug into the current socket. Kind of messy and expensive. The more
that I thought of just doing a GIME replacement or improvement the
more it really seems that cleanest way is to do the CPU, improved
GIME, DAC and the PIA chips in one FPGA. IF one were to really expand
the functions of the GIME to say expanded MMU and VGA capability and
keep backward socket capability, that becomes more limiting of what an
FPGA can do. 

I have so far have about 30% coded. Problem right now is that I may
not have ability to test 15 KHz horizontal sync frequency video modes
 until later this summer or early fall. The current monitor I have for
my PC will only handle down to 30 KHz line frequency. Right now I have
invision using sram for memory and not drams. Even 1 meg density drams
are getting scare and 512K x 8 20 nS srams are plentiful and under $10
each. I have the basic timing circuits in mind for standard IBM and
Vesa VGA and SVGA capabilities. Looking how to do 16 and 8 bit color. 

As the speed of the sram indicates, I am hoping that I can get the CPU
and GIME II to work at higher than 25 MHz. One critical thing is that
I don't want to trade off to much area on the FPGA for speed. The key
to breaking 25 Mhz will be getting the ALU to work at speeds greater
than 65 MHz. I also want to make the CPU to have as much of the 6309
expanded opcodes as I can. Doing that will take time and care to do
the microcode for the functions. 

Pluggin along bit by bit

james
 






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