32
Table 2.2
MCU Operating Mode Selection* (ROMless and Mask ROM Versions)
MCU
CPU
External Data
Bus
Operating
Mode
MD2
MD1
MD0
Operating
Mode
Description
On-Chip
ROM
Initial
Width
Max.
Width
0
0
0
0
—
—
—
—
—
1
1
Advanced
Expanded mode
with on-chip
Disabled
16 bits
16 bits
2
1
0
ROM disabled
8 bits
16 bits
3
1
—
—
—
—
—
4
1
0
0
Advanced
Expanded mode
with on-chip
ROM enabled
Enabled
8 bits
16 bits
5
1
External ROM
activation
expanded mode
16 bits
16 bits
6
1
0
with on-chip
ROM enabled
8 bits
16 bits
7
1
Single-chip
activation mode
with on-chip
ROM enabled
—
16 bits
Note:
*
Only modes 1 and 2 are available in the ROMless version.
The CPU’s architecture allows for 4 gigabytes of address space, but the H8S/2678 Series chip
actually accesses a maximum of 16 Mbytes.
Modes 1, 2, and 4 to 6 are externally expanded modes that allow access to external memory and
peripheral devices.
The externally expanded modes allow switching between 8-bit and 16-bit bus modes. After
program execution starts, an 8-bit or 16-bit address space can be set for each area, depending on
the bus controller setting. If 16-bit access is selected for any one area, 16-bit bus mode is set; if 8-
bit access is selected for all areas, 8-bit bus mode is set. Pin functions depend on the operating
mode.
In the single-chip activation externally expanded mode, it is possible to switch between externally
expanded mode and single-chip mode. Immediately after a reset, the chip starts up in single-chip
mode, but after the start of program execution, it is possible to change to externally expanded
mode by setting the EXPE bit in the system control register (SYSCR) accordingly. Pin functions
depend on the operating mode.
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