[Coco] SAM and RAM refresh.

[Phill Harvey-Smith]

Hi,

I've just as promised had a poke around the sam with a scope when it's 
in double speed mode (poke &hffd9,1), and found the following :

The CAS and RAS signals seem to always toggle at 1.7MHz, irrespective of 
  the speed selected, in normal mode what happens is that the CPU and 
video access the ram on alternate CAS/RAS cycles, as expected, and in 
double speed the CPU gets an access every cycle and the VDG gets none.

So as documented on the SAM data sheet, the CPU literally steals all of 
the VDG access, and so no refresh happens.

The interesting thing I found was that the RAS/CAS still toggle even if 
the CPU is accessing ROM, I guess this is doing a dummy read and the 
gating on the ram data out prevents the read data from reaching the CPU 
data bus.

I suppose this could be exploited with some extra circuitry, to do a 
refresh when the CPU is accessing ROM, but this would of course only be 
of use to BASIC or ROM based programs, and could not be used in a RAM 
based os such as OS-9 or Flex.

Cheers,

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.