[Coco] MCM68766

[Chuck Youse]

On Sun, 2008-07-13 at 23:21 -0400, jdaggett at gate.net wrote:
> Mike
> 
> You need 250nS access time for the ROM memory for the Coco to run at 
> 1.78MHz period. On the ROMs and static ram the access time and the cycle 
> time are equivalent. The maximum access time is the fastest that the ROM 
> can operate at. The inverse of 350nS is about 2.85MHz. Since the Coco 
> uses IDMA to access both data and video memory, at 1.78MHz half of the 
> 560 nS cycle time is used to access data/program memory. The other half is 
> used to access video memory.  That is 260nS for a read cycle at 1.78MHz. 
> This means in a Coco system the fastest clock for 350nS ROMs is about 
> 1.425 MHz. Like I said before they work fine with the clock at 0.89MHz 

I think that you're perhaps over-complicating the situation; the
access-interleaving done by the GIME really only applies to the dynamic
RAMs -- the GIME does not get in the middle of bus cycles for ROMs or
I/O regions, except as an address-decoder (S[2:0]).

The access time of the I/O ports and EEPROMs must be < ~280ns, not
because of any interleaving, but because an access begins on the
positive edge of the E clock, and is completed on the negative edge
(more or less).  At 50% duty cycle that means the access time is roughly
1/2 the period of the system clock.

Or did I miss something?

C.