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RE: [OM] Timer Mod for OM-2S/OM-4T

Subject: RE: [OM] Timer Mod for OM-2S/OM-4T
From: Olaf Greve <Ogreve@xxxxxxx>
Date: Wed, 26 May 1999 12:02:52 +0200
Hi,

> analog chip. The chip runs off a xtal controlled
> clock which runs at 32kHz external that is
> multiplied up to 500kHz for the processor 
> itself. In theory you could up the clock frequency
> to speed things up when the timer sw is 
> selected and then once the exposure starts (using
> say the signal for the mirror or first shutter)
> switch back to the normal xtal, however this is 
> probably too much work.

Exactly, even though theoretically your idea might work, the practical side
of it would be next to undoable.

Suppose the following scenario:
------------------------------
Assume there is some glue logic installed which already makes the
distinction between running in "self timer mode" and "normal mode". If this
is the case, in theory you could reprogram the thing, add an additional
clock source which is 6x times faster than the one normally used (i.e.
192KHz instead of 32KHz), and let the glue logic simply function as a
selector between the two different external clock sources. 

Doing this will incur an inevitable delay penalty in your clock signal.
Since you state above that the OBC of these cameras uses internal
multiplication (up to 500KHz) to achieve the actual clock speed, you'll have
a 2 micro second cycle time in your processor. This is relatively slow and
consequently allows for components of which timing doesn't come down to
nanoseconds (as is the case when having to work with xxx MHz processors). So
probably the shift in the clocksignal doesn't interfere with the timing of
the other signals that go to the processor. However, in the case that the
shift in the clock signal causes the processor to have incorrect timing, you
will end up with a malfuncioning system (as the processor will no longer
"recognise all commands" because it doesn't have the proper signals timely
available). Again, because of the slow processor speed, this probably will
not be an issue. However, the internal processor might form more of an issue
if all of a sudden its clock speed goes from (a multiplied) 500KHz to 3MHz.
 
Even assuming that none of these timings etc. form any issue, you will still
need to know how the glue logic works, and reprogram it. If the glue logic
is not present you will need to add it and program it. Also, you will either
need to add a 192KHz clock source (along with the proper wiring), or replace
the old source to overclock the processor 6x times! Obviously, this will
become the main issue, along with all the PCB reworking that would be
needed.

Of course there are some other possible scenarios and/or "solutions", but I
think none of them is feasible, unless the 32KHz clock source is ONLY used
for determining the length of the selftimer cycle (and NOT as the driving
clock for the processor), but from what you state above I think that is not
the case. However, should this be the case after all, theoretically,
replacing that clock source for the faster one should suffice, if and only
if the processor can then still correctly "count" the XTAL clock cycles.

Just my 2 cents from an engineering point of view.
Anyone want to go down this road?

Cheers!
Olaf


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