README on the IOBARRIER for CardEngine IO
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Due to an unfortunate oversight when the Card Engines were designed,
the signals that control access to some peripherals, most notably the
SMC91C9111 ethernet controller, are not properly handled.

The symptom is that back to back IO with the peripheral returns
unreliable data.  With the SMC chip, you'll see errors about the bank
register being 'screwed'.

The cause is that the AEN signal to the SMC chip does not transition
for every memory access.  It is driven through the CPLD from the CS7
line of the CPU's static memory controller which is optimized to
eliminate unnecessary transitions.  Yet, the SMC requires a transition
for every access.  The Sharp website has more information on the
effect of this power conservation feature on peripheral interfacing.

The solution is to follow every access to the SMC chip with an access
to another memory region that will force the CPU to release the chip
select line.  Note that it is important to guarantee that the access
will force the CPU off-chip.  We map a page of SDRAM as if it were an
uncacheable IO device and read from it after every SMC IO operation.

  SMC IO
  BARRIER IO

You might be tempted to believe that we must access another device
attached to the static memory controller, but the empirical evidence
indicates that this is not so.  Mapping 0x00000000 (flash) and
0xc0000000 (SDRAM) appear to have the same effect.  Using SDRAM seems
to be faster.