[TI-H] Re: TI-89 HW2 overclocking


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[TI-H] Re: TI-89 HW2 overclocking




On Mon, 24 Feb 2003 16:54:44 -0500 "B.A. Baracus"
<babaracus@bellsouth.net> writes:
> I don't see anything called Chip Select, however, under Write Cycle, 
> I see the following:
> 
> Write Cycle Time                        min 85ns
> Write Pulse Width                        min 60ns
> Chip Enable to End of Write        min 75ns
> 
> Is one of these the relevant value that is being exceeded by the 
> calculator
> that is preventing overclocking beyond 15~20MHz?

Now, I'm not 100% sure, but this seems to say that the fastest you can
write is once every 85ns (that is, one write cycle takes 85ns, after
which you can begin another) and you need to have the chip enable and all
the proper signals stable for at least 60ns.  The timing diagrams seem to
agree, but again I'm not sure.

It looks like (dunno, 68k ain't my thing, I'm going by figure 4-2 in the
68k user's manual), if the data strobe is used as chip enable, for a read
it is enabled for 2 processor clocks, while for write it's only 1. 
(although with a lower reliability it could be a little longer because of
rise/fall time and because it'll go a little outside of its rating)

This would mean that the processor clock period needs to be at least
about 60ns for writes to work (min write pulse width), even if it only
does a r/w operation at most once every 4 clocks (which would limit the
proc. clock period to 85ns/4 = 22ns)

60ns min processor clock period = 60 * 10^-9 s
1/(60 * 10^9 s) = 16.7 * 10^6 1/s = 16.7 MHz max clock frequency

My apologies is this is entirely wrong and misleading, it's been a while
since I've done anything with electronics...

-rabidcow


   

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