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XC4000: What values do rams contain immediately after configuration?
It is impossible to know what values are contained within
a RAM block used in an XC4000 design until data has been
written to the RAM after configuration has ended. It is also
impossible to set an initialization value for a RAM within
the configuration bitstream. The reason for this is that
the write enable line of the RAM may experience transitions
as the programmable interconnect points that determine its
routing are turned on by the configuration process. Because
of this, spurious values on the data input of the RAM may
be written to it.
For the XC4000 devices the only way to set the initial value of
the ram is to do a write to the ram after configuration, and
set the values at that time. The XC4000E devices allow you
to initialize the RAM to a specific state during configuration.