Mode and Resource Mapping Registers
MC68HC812A4 Data Sheet, Rev. 7
Freescale Semiconductor
61
5.4.5 Miscellaneous Mapping Control Register
Additional mapping controls are available that can be used in conjunction with memory expansion and
chip selects.
To use memory expansion, the part must be operated in one of the expanded modes. Sections of the
standard 64-Kbyte memory map have memory expansion windows which allow more than 64 Kbytes to
be addressed externally. Memory expansion consists of three memory expansion windows and six
address lines in addition to the existing standard 16 address lines. The memory expansion function
reuses as many as six of the standard 16 address lines. Usage of chip selects identifies the source of the
internal address.
All of the memory expansion windows have a fixed size and two of them have a fixed address location.
The third has two selectable address locations.
Read: Anytime
Write: Once in normal modes; anytime in special modes
EWDIR — Extra Window Positioned in Direct Space Bit
This bit is only valid in expanded modes. If the EWEN bit in the WINDEF register is cleared, then this
bit has no meaning or effect.
1 = If EWEN is set, then a 1 in this bit places the EPAGE at $0000–$03FF.
0 = If EWEN is set, then a 0 in this bit places the EPAGE at $0400–$07FF.
NDRC — Narrow Data Bus for Register Chip-Select Space Bit
This function requires at least one of the chip selects CS3–CS0 to be enabled. It effects the external
512-byte memory space.
1 = Makes the register-following chip-selects (2, 1, 0, and sometimes 3) active space (512-byte
block) act the same as an 8-bit only external data bus. Data only goes through port C externally.
This allows 8-bit and 16-bit external memory devices to be mixed in a system.
0 = Makes the register-following chip-select active space act as a full 16-bit data bus. In the narrow
(8-bit) mode, NDRC has no effect.
Address: $0013
Bit 7
654321
Bit 0
Read:
EWDIR
NDRC
000000
Write:
Reset:
00000000
Figure 5-5. Miscellaneous Mapping Control Register (MISC)