h8s-2138 Renesas Electronics Corporation., h8s-2138 Datasheet - Page 200

• Manufacturer Part Number: h8s-2138
• Description: Renesas 16-bit Single-chip Microcomputer H8s Family/h8s/2100 Series
• Manufacturer: Renesas Electronics Corporation.

Section 6 Bus Controller
6.4
6.4.1
The basic bus interface enables direct connection of ROM, SRAM, and so on.
The bus specifications can be selected with the AST bit, and the WMS1, WMS0, WC1, and WC0
bits (see table 6.3).
6.4.2
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.
These group only have an upper data bus, and only 8-bit access space alignment is used. In these
group, the upper data bus pins are designated D7 to D0.
8-Bit Access Space: Figure 6.3 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 access is performed as two byte accesses,
and a longword access, as four byte accesses.
Rev. 4.00 Jun 06, 2006 page 146 of 1004
REJ09B0301-0400
Byte size
Word size
Longword size
Basic Bus Interface
Overview
Data Size and Data Alignment
Figure 6.3 Access Sizes and Data Alignment Control (8-Bit Access Space)
1st bus cycle
2nd bus cycle
1st bus cycle
2nd bus cycle
3rd bus cycle
4th bus cycle
D15
Upper data bus
D8 D7
Lower data bus
D0