[Coco] Dynamic RAM Q

[Phill Harvey-Smith]

Hi CoCo/Dragon friends,

This may be a little long and rambling, but bear with me.....

I am currently working on a memory expansion board for the CoCo 1/2 & 
Dragon computers, that will hopefully allow me to have up to 1 MB of 
RAM, using an old PC 1MB SIMM, as I have a whole box of these. I 
currently have it emulating the Extra ram part of the Dragon Plus 
expansion system that gave the Dragon an extra 64K of ram that could be 
paged into the bottom 32K of the memory map. This seems to work ok.

Currently I have the Z0..7 lines from the SAM connected to A0..7 on the 
SIMM, the additional A8..11 from the SIMM are connected to a CPLD, so 
that I can toggle the extra address lines as needed. Currently the logic 
in the CPLD consists of a latch that outputs extra values to these lines 
  to page in the extra RAM. The CPLD is also connected to the 6809 
Address & Data busses, as well as E,Q, R/W, WE etc. I will be adding a 
small SRAM to act as the DAT, as was done in the Dragon Beta prototype, 
I guess this is buried in the GIME in the CC3.

My (possible) Problem, is that for the next stage to fully address the 
1M, I will need to be able to put different values on A8 & 9, for Row 
and column. However the diagrams on all the data sheets I have seen seem 
to suggest that the address lines need to be setup before the RAS or CAS 
transition and remain stable afterwards.

However if this is the case there will be a problem, as I will need to 
switch the lines between the RAS and CAS transitions, which could be a 
problem, as there does not seem to be any signal I could utilise to make 
this switch.

The other thing I did notice from the data sheet for the chips on my 
SIMM (HYB511000), is that the setup time for RAS and CAS has a minimum 
of 0, the important thing seems to be the hold time, after the RAS or 
CAS transition, so it would seem from this that I could just us a 
combination of the RAS and CAS signals to decide which bits I need to 
output. So :-

RAS	CAS	A8..9
1	1	high Z
0	1	Row bits
0	0	Col bits
1	0	Col bits.

I have done a brief test where I output 00, for the row bits, and the 
values from my latch for the Col bits and this seems to work.

I know some of the other people on this list have worked with things 
like this before, and may be able to shed some light on this.

BTW thanks whoever it was that suggested using CAS before RAS to 
refresh, it has really made this thing MUCH easier, as I can just swap 
RAS and CAS when E and HS are both low, and fakeup RAS from Q (as CAS is 
suppressed by the SAM for refresh cycles).

One final curious thing about this setup is this, if I turn the machine 
on, type a 2 line 10 Print "Phill was here "; 20 Goto 10 type program, 
and run it, it runs. If I then turn the machine OFF for 5 seconds and 
then back on again, and type RUN it's still there !!!! I have also 
noticed this on some CoCo 1/2 models too, so much for Dynamic ram 
needing to be refreshed :) :) :)

Cheers,

Phill.

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

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