[Coco] MCM68766

[jdaggett at gate.net]

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 

Motorola speced that part at 250nS and 350nS. If it works reliably at the 
faster speed, it would have been marked that. Even if it had an access time 
of 300nS, which is faster than 350nS, it is still to slow for 1.78MHz clock.

james

On 13 Jul 2008 at 14:47, Mike Pepe wrote:

> > -----Original Message-----
> > From: coco-bounces at maltedmedia.com [mailto:coco-
> > bounces at maltedmedia.com] On Behalf Of jdaggett at gate.net
> > Sent: Sunday, July 13, 2008 11:46 AM
> > To: Joe; CoCoList for Color Computer Enthusiasts
> > Subject: Re: [Coco] MCM68766
> > 
> > Joe
> > 
> > They are useful as long as you keep the Clock at 0.89MHz. They are
> > to slow for the 1.78MHz clock. As for programming them most older,
> > 80's vintage, EPROM programers will suffice.
> > 
> > james
> > 
> 
> Keep in mind that this rating is the maximum guaranteed access time.
> It's quite possible the device may be faster-it's just guaranteed not
> to be any slower.
> 
> Regardless, though, it'll work at the stock clock rate, as pointed out
> above.
> 
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