Acorn Second Processors

[Eelco Huininga]

> > Hi,
> > 
> >  >> Changing the subject slightly, IIRC the original "Acorn 6502
> >  >>Second Processor" cheese-wedge was just that, a faster 6502?
> >  >
> >  > Actualy, it's a 3MHz 65C02.
> > 
> > Ah, thanks for that....it's been a few years since I had the lid off mine.
> > 
> > 
> >  >>....problems (in fact, total failure IIRC?) when I tried running
> >  >>a 65c02 in a Model B sometime in the early 90's?
> >  >
> I too recently tried that, with three new 65c02's. The BBC would have 
> none of it : it only booted with the NMOS 6502
> 
> 							Jos

IIRC this was caused by a slight timing difference of the Phi2 signal.
This could be solved by placing two 220pF ceramic capacitors over pins
7 and 11 of IC29 and IC37. This would fix the Beeb's booting problem, but
unfortunately this would create a DRAM timing problem for Solidisk's
96k Sideways RAM expansion for the 32k SWR board. Took me (literally)
years to figure that out!

One of my 'when-i-have-time' things is experimenting with the capacitor's
value to see if I can find the right value :-)


Cheers,
Eelco