> ] May I ask you if you could tell me what is the value of the pull up
> ] resistors for the 3 upper bits of the mapper registers?
> I'm sorry but I do not know the exact values.
The external pull-up resistors, those that give the databus its high
state if it is not driven from any source, are well dimensioned.
Otherwise, the databus couldn't be pulled down, even by the engine
itself. Besides, if you mean the internal pull-ups in the driver
section: these are non-linear resistors (transistors), so there is
no single value, but an U-I curve .
> I only know that most external mappers are not able to pull the
> voltage down close enough to zero.
>
> And the change of blowing up your engine is very small indeed, just because the
>signals don't last that long.
> The current will only be flowing during the few clock cycles of the
> read from the mapper.
Has anyone ever actually blown up a S1990 with an internal/external
mapper?
If this chip happens to be nmos (as is the v9938), it may be possible
to pull the databus down to zero, even if it is driven high by the
engine. The output current in that case is limited by the internal
pull-up transistor of the driver section of the mapper in the S1990.
Greetings,
Jerome Borsboom
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