Date   : Mon, 06 Oct 2008 21:19:40 +0100
From   : mfirth@... (Michael Firth)
Subject: New 6502 second processors?

----- Original Message ----- 
From: "Phil Blundell" <philb@...>
To: "Michael Firth" <mfirth@...>
Cc: <bbc-micro@...>
Sent: Monday, October 06, 2008 8:45 PM
Subject: Re: [BBC-Micro] New 6502 second processors?


> On Mon, 2008-10-06 at 20:14 +0100, Michael Firth wrote:
>> Has anyone given any thought to producing replacement internal or
>> external 6502 second processors?
>
> I did think about this at one point, and I actually got as far as
> building some PCBs with the appropriate circuitry on.  The idea was to
> put the 6502 plus ULA emulation into a single FPGA - some kind of
> Spartan-3 if I remember correctly, clocked at somewhere around 40MHz.
> Aside from that you just needed an external SRAM, an EPROM, some bus
> level translators and the power supply circuitry.  I think I also ended
> up having to use a small XC9500 PLD to load the image into the FPGA, for
> reasons which escape me at present.
>
> Annoyingly though it turned out that I had gotten the PLD pinout
> hopelessly wrong: I had somehow laid the board out for a TQFP device but
> used the pin assignments from the PLCC part, or vice versa.  In any case
> it would have required an inordinate amount of fiddly hand-wiring to
> make the device fit, and I don't think I ever had the patience to do
> that.
>
> That was a couple of years ago now, and I suspect most of the parts I
> used at the time are probably discontinued.  I'm pretty sure the RAM, in
> particular, is impossible to buy now.  I keep thinking about having
> another crack at that project but it would probably need to be more or
> less redesigned from the ground up.
>
> p.
>
I guess for an FPGA based solution, the Xilinx XC3S50AN would be quite good 
(assuming it is large enough), as it is 'self programming' as it has a built 
in PROM. Unfortunately the larger parts in the family are only available as 
BGAs, which would make small scale production / testing very tricky.

I would have thought that it should be possible to incorporate the EPROM 
inside the FPGA too, assuming its kept as a clone of the original Acorn ROM 
or similar, as that's only 2kbytes, so would fit inside the FPGA block ram 
relatively easily, and reduce the component count even further.

It looks as though small enough SRAMs may be hard to come by these days. It 
looks as though Cypress do a 128k x 8 device (CY7C1018DV33) that would 
probably be suitable if you didn't mind only using half of it!

Did you get anywhere with pulling together the firmware building blocks for 
the FPGA, or had you only got to the hardware design?

Michael