Date   : Sat, 30 Aug 2008 21:31:11 +0200
From   : kortink@... (John Kortink)
Subject: FRAM....

On Wed, 27 Aug 2008 22:46:28 +0100, "Mark Haysman" wrote:

>Hi Folks.
>
>I've been playing with some FRAM, namely the Ramtron FM1808. I've got a B 
>which runs fine with the 62256 SWR mod in socket 15, however, it plays up 
>when I pop a FRAM in there.
>
>If I write a ROM to it and reset the machine, it crashes. I initially 
>thought that it's the way the FRAM latches addresses on the /CE pin maybe 
>that's causing it not to write correctly, but it appears not.

Think again. nCE on the model B is just a plain
combinatorial address decode (A[15:14] == 10 &&
ROMSEL[3:0] == whatever) which means it is only
stable in the data transfer phase of the 6502
cycle (roughly speaking on '2MHZE' in the model
B schematic).

So you're asking for trouble if you don't validate
nCE with 2MHZE (or, therefore, nOE, which has 2MHZE
factored in). Without that, it may work some of the
time, but when the first glitch on nCE comes (and
there will be many) you're screwed.


John Kortink

-- 

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