Re: 6569 luminances

----- Original Message ----- 
From: "Gerrit Heitsch" <gerrit@laosinh.s.bawue.de>
To: <cbm-hackers@musoftware.de>
Sent: Monday, May 14, 2012 1:34 PM
Subject: Re: 6569 luminances

> With a CMOS-EPROM it's not much, but there are still a few NMOS-EPROMs
> around that could get used and with them it makes a difference, at least
> in heat output.

Oh, come on; unless you insist on using some really ancient and obsolete
versions (and can program them) even NMOS EPROMs still only draw more or
less the same power as the ROMs you're replacing, and we are after all
talking about replacing 3 chips with one.

 >>   * I was worried that the delay of enabling the ROM and it's output
>>     drivers would cause timing issues. On a 250nS 27C64, it can take up
>>     to 350nS from !CE to data valid. That's compared to 120nS for !OE to
>>     data valid.
>
> Where did you get the 350ns for a  250ns EPROM?

Yeah, AFAIK the speed rating of an EPROM refers to the /CE to valid output
delay so it would be 250ns, but in any case the /OE to output delay is 
always shorter AFAIK and sometimes substantially so.

So, all in all, considering that a CMOS EPROM will almost certainly draw
substantially less power than the NMOS ROMs you're replacing and that using
/OE to select will give you a valid output more quickly than using /CE, I'd
say that using /OE is actually 'the better way' ;-)

m


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