Date   : Thu, 04 May 2006 07:43:44 +0100
From   : Sprow <info@...>
Subject: Re: Western Design Center W65C02S

In article <44592724.1020401@...>,
   Peter Rendle <pete@...> wrote:

[WDC 65C02 processor]

> >> [...] would I be best building one into a second processor and 
> >> running it at 14Mhz? ( Probably tricky!!)
> >>     
> > The intimate relationships between timing on the BBC means you can't 
> > easily jack up the CPU rate without melting the VIAs/patching 
> > software timing loops in filing systems/getting video coming out 
> > at the wrong rate/etc...
> >
> > I'd suggest using one in a second processor would be simpler since
> > you only
> > have the RAM timing to worry about then, and frankly there's no reason to
> > use DRAM for 32k now - a 70ns SRAM will suffice.
> >
> > Of course I'd have to sway you towards an ARM coprocessor in favour of
> > trying to squeeze a WDC part onto the Tube,
>
> The ARM coprocessor does look very sexy! (Its on my birthday list)
>
> Seeing you are obviously the person to ask.....
>
> Would a normal Tube ULA cope with interfacing to a faster coprocessor?  
> And if not are there any alternatives?

The normal ULA is specced for up to 4MHz 6502 timing, so it would work with
a WDC part but you'd have to slow down during a Tube access in much the same
way as the main 6502 slows down during a 1MHz bus access.

The alternative is to not use the ULA at all and use a VIA/PIO/etc... as a
bus isolator and interrupt source much like the Torch Z80 did with an 8255,
Sprow.