
EZ-USB FX2 Technical Reference Manual
Page 5-8 EZ-USB FX2 Technical Reference Manual v2.1
Be careful to check the access time of external Flash or other code memory in this mode. The FX2
can stretch its RD and WR strobes to compensate for slow
data
memories, but it does not have
the capability to stretch its PSEN signal to allow for slow code memories. At 48 MHz, an external
code-memory chip must have an access time of approximately 44 ns or shorter (access-time
parameters are given in the CY7C68013 data sheet).
5.5 “Von-Neumannizing” Off-Chip Program and Data Memory
The 128-pin FX2 package provides a 16-bit address bus, an 8-bit data bus, and memory control
signals PSEN
, RD, and WR. These signals are used to expand the FX2’s External Program and/or
External Data memory.
As described in the previous section, the FX2 gates the RD
and WR signals to exclude selection
of off-chip data memory in the range occupied by the on-chip memory. The PSEN
signal is also
available on a pin for connection to off-chip code memory.
In some systems, it may be desirable to combine off-chip program and data memory, just as the
FX2 combines its on-chip program/data Main RAM. These systems must logically OR the PSEN
and RD strobes to qualify the off-chip memory’s chip enable and output enable signals. To save
the external logic which would normally be needed, FX2 provides two additional control signals,
CS
and OE. The equations for these active-low signals are:
CS
= RD + WR + PSEN
OE = RD + PSEN
Because the RD, WR, and PSEN signals are already qualified by the addresses allocated to off-
chip memory, the added strobes CS
and OE strobes are active only when the FX2 accesses off-
chip memory.
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