HD64F2149YV datasheet(176/1075 Pages) RENESAS

176 / 1075 page

Section 6 Bus Controller

Rev. 3.00 Jan 18, 2006 page 148 of 1044

REJ09B0280-0300

6.4

Basic Bus Interface

6.4.1

Overview

The basic bus interface enables direct connection of ROM, SRAM, and so on.

The bus specifications can be selected with the ABW bit, the AST bit, and the WMS1, WMS0,

WC1, and WC0 bits (see table 6.3).

6.4.2

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 6.3 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.

D15

D8 D7

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 6.3 Access Sizes and Data Alignment Control (8-Bit Access Space)

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