
Chapter 11. CPU Introduction Page 11-11
11.13 External Address/Data Buses
The 128-pin version of the FX2 provides external, non-multiplexed 16-bit address and 8-bit data
buses. This differs from the standard 8051, which multiplexes eight pins among three sources:
I/O port 0, the external data bus, and the low byte of the external address bus.
A standard 8051 system with external memory requires a demultiplexing address latch, strobed by
the 8051 ALE (Address Latch Enable) pin. The external latch is not required by the FX2 chip, and
no ALE signal is provided. In addition to eliminating the need for this external latch, the non-multi-
plexed FX2 bus saves one cycle per memory-fetch and allows external memory to be connected
without sacrificing I/O pins.
The FX2 is the sole master of the bus, providing read and write signals to the off-chip memory. The
address bus is output-only, and cannot be floated.
11.14Reset
The various FX2 resets and their effects are described in
Chapter 7, "Resets"
.
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