
M37902FCCHP, M37902FGCHP, M37902FJCHP
MITSUBISHI MICROCOMPUTERS
28
SHINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
G
Burst ROM access
When ROM supporting the burst ROM access has been allocated to
area CS
i
, the burst ROM access can be specified. The burst ROM
access is specified by each burst ROM access select bit of the CS
i
control register L (bit 5 at addresses 80
16
, 82
16
, 84
16
, 86
16
). The
burst ROM access is valid only when the external data bus width =
16 bits with an instruction prefetched. In the other cases, the normal
access is performed regardless of the contents of the burst ROM ac-
cess select bit. The burst ROM access can be specified only in the
case of
in Figure 18.
Figure 20 shows a waveform example at burst ROM access.
When an instruction is prefetched from the burst ROM, 8 bytes are
fetched starting from an 8-byte boundary (the low-order 3 bits of ad-
dress, A
2
, A
1
, A
0
=
“
000
”
) in waveform (a). When branched, regard-
less of the 8-byte boundary of the branch destination address,
access starting from the 4-byte boundary (the low-order 2 bits of ad-
dress, A
1
, A
0
=
“
00
”
) is performed in waveform (b). Once the 8-byte
boundary has been selected, instructions will be prefetched in wave-
form (a) until a branch.
Fig. 20 Waveform example at burst ROM access
Note:
The above is applied when 1 bus cycle = 2
φ
.
(b)
External address bus
(A
0
to A
23
)
RD
External data bus
(D
0
to D
7
)
Data
(Instruction)
External data bus
(D
8
to D
15
)
(Instruction)
Data
(Instruction)
(Instruction)
Address
Address
(a)
External address bus
(A
0
to A
23
)
RD
External data bus
(D
0
to D
7
)
Data
(Instruction)
External data bus
(D
8
to D
15
)
Data
(Instruction)
Data
(Instruction)
Data
(Instruction)
φ
1
Address
Address
Address
Address
Data
(Instruction)
Data
(Instruction)
Data
(Instruction)
Data
(Instruction)
φ
1
A
A
Note:
The above is applied when 1 bus cycle = 2
φ
.
Notes 1:
The burst ROM access is selected by the burst ROM access select bit (bit 5 at addresses 80
16
, 82
16
, 84
16
, 86
16
).
2:
The burst ROM access can be selected only in the case of
in Figure 18.