Datasheet

Section 6 Bus Controller
REJ09B0140-0900 Rev. 9.00 Page 127 of 846
Sep 16, 2010
H8S/2215 Group
6.5.3 External Address Space Access Timing
The external address space is accessed with an 8-bit or 16-bit data bus width in a two-state or
three-state bus cycle. In three-state access, wait states can be inserted. For further details, refer to
section 6.6.3, Basic Timing.
6.6 Basic Bus Interface
The basic bus interface enables direct connection of ROM, SRAM, and so on.
6.6.1 Data Size and Data Alignment
Data sizes for the CPU and other internal bus masters are byte, word, and longword. The bus
controller has a data alignment function, and when accessing external space, controls whether the
upper data bus (D15 to D8) or lower data bus (D7 to D0) is used according to the bus
specifications for the area being accessed (8-bit access space or 16-bit access space) and the data
size.
8-Bit Access Space: Figure 6.8 illustrates data alignment control for the 8-bit access space. With
the 8-bit access space, the upper data bus (D15 to D8) is always used for accesses. The amount of
data that can be accessed at one time is one byte: a word transfer instruction is performed as two-
byte accesses, and a longword transfer instruction, as four-byte accesses.
D15 D8 D7 D0
Upper data bus Lower data bus
Byte size
Word size
1st bus cycle
2nd bus cycle
Longword
size
1st bus cycle
2nd bus cycle
3rd bus cycle
4th bus cycle
Figure 6.8 Access Sizes and Data Alignment Control (8-Bit Access Space)
16-Bit Access Space: Figure 6.9 illustrates data alignment control for the 16-bit access space.
With the 16-bit access space, the upper data bus (D15 to D8) and lower data bus (D7 to D0) are
used for accesses. The amount of data that can be accessed at one time is one byte or one word,
and a longword transfer instruction is executed as two word transfer instructions.