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SH7137 Datasheet, PDF (263/1160 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer SuperH™ RISC engine Family | |||
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Section 9 Bus State Controller (BSC)
9.5.10 Access to External Memory by CPU
Table 9.10 shows the number of cycles required for access to the external memory by the CPU. As
the table shows, the number of cycles varies with the clock ratio, the access size, the external bus
width of the LSI, and the setting for wait insertion. For details on the wait-insertion setting, see
section 9.4, Register Descriptions.
Table 9.10 Number of External Access Cycles
External Access
Bus Width Size
Write/Read
Number of Access Cycles
8 bits
Byte
Write
(1 + n) Ã IÏ + (3 + m) Ã BÏ
Read
(1 + n) Ã IÏ + (3 + m) Ã BÏ + 1 Ã IÏ
Word
Write
(1 + n) Ã IÏ + (3 + m) Ã BÏ + 1 Ã (2 + o) Ã BÏ
Read
(1 + n) Ã IÏ + (3 + m) Ã BÏ + 1 Ã (2 + o) Ã BÏ+ 1 Ã IÏ
Longword Write
(1 + n) Ã IÏ + (3 + m) Ã BÏ + 3 Ã (2 + o) Ã BÏ
Read
(1 + n) Ã IÏ + (3 + m) Ã BÏ + 3 Ã (2 + o) Ã BÏ+ 1 Ã IÏ
Notes: n:
m, o:
When IÏ:BÏ = 8:1, n = 0 to 7
When IÏ:BÏ = 4:1, n = 0 to 3
When IÏ:BÏ = 3:1, n = 0 to 2
When IÏ:BÏ = 2:1, n = 0 to 1
When IÏ:BÏ = 1:1, n = 0
m: Wait setting, o: Wait setting + idle setting
For details, see section 9.4, Register Descriptions.
Synchronous logic and a layered bus structure have been adopted for this LSI circuit. Data on each
bus are input and output in synchronization with rising edges of the corresponding clock signal.
The L bus and I bus are synchronized with the IÏ and BÏ clocks, respectively. Figure 9.13 shows
an example of the timing of write access to a word of data over the external bus, with a bus-width
of 8 bits, when IÏ:BÏ = 2:1. Once the CPU has output the data to the L bus, data are transferred to
the I bus in synchronization with rising edges of BÏ. There are two IÏ clock cycles in a single BÏ
clock cycle when IÏ: BÏ = 2:1. Thus, when IÏ: BÏ = 2:1, data transfer from the L bus to the I bus
takes (1 + n) Ã IÏ (n = 0 to 1) (2 Ã IÏ is indicated in figure 9.13). The relation between the timing
of data output to the L bus and the rising edge of BÏ depends on the state of program execution.
Data output to the I bus are transferred to the external bus after one cycle of BÏ. External access to
each data takes at least two cycles, and this can be prolonged by the BSC register settings (m and
o in the formulae for number of access cycles). In the case shown in figure 9.13, since n = 1, m =
0, and o = 0, access takes 2 Ã IÏ + 3 Ã BÏ + 2 Ã BÏ.
Rev. 2.00 Sep. 10, 2008 Page 237 of 1130
REJ09B0402-0200
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