M68HC11K Datasheet, PDF (242/290 Pages) Freescale Semiconductor, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Memory Expansion and Chip Selects
11.4.3 General-Purpose Chip Selects
The general-purpose chip selects are the most flexible and
programmable of the chip-select signals. They can access any memory
in the expanded 1-Mbyte address space. Polarity of active state, E valid
or address valid, size, and starting address are all programmable. Clock
stretching can be set from zero to three cycles. Both signals can be
programmed to drive CSPROG, and GPCS1 can be configured to drive
GPCS2. In addition, each signal can follow a window; for instance, be
asserted whenever the CPU address falls within a selected memory
expansion window regardless of the state of the expanded address
lines.
There are two registers for each of the general-purpose chip select
signals:
• The control register, GPS1C or GPS2C, determines the GPCS’s
active signal polarity, its valid time, which of the other chip-select
signals it can drive, and either the size of the memory it enables or
which window it follows.
• The address register, GPS1A or GPS2A, programs the
chip-select’s starting address; valid bits in this register are
determined by the size of the address range selected by the
control register.
In addition, the MMSIZ register contains a bit for each GPCS which
determines whether it is driven by the CPU’s 64-Kbyte address lines or
the expansion address lines.
Technical Data
242
Memory Expansion and Chip Selects
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M68HC11K Family
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