
M37902FCCHP, M37902FGCHP, M37902FJCHP
MITSUBISHI MICROCOMPUTERS
28
SHINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
q Burst ROM access
When ROM supporting the burst ROM access has been allocated to
___
area CSi, the burst ROM access can be specified. The burst ROM
___
access is specified by each burst ROM access select bit of the CSi
control register L (bit 5 at addresses 8016, 8216, 8416, 8616). 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, A2, A1, A0 = “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, A1, A0 = “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
RD
External data bus
Data
(Instruction)
External data bus
Data
(Instruction)
Data
(Instruction)
Data
(Instruction)
Address
(a)
External address bus
(A0 to A23)
RD
External data bus
(D0 to D7)
Data
(Instruction)
External data bus
Data
(Instruction)
Data
(Instruction)
Data
(Instruction)
φ1
Address
Data
(Instruction)
Data
(Instruction)
Data
(Instruction)
Data
(Instruction)
(D8 to D15)
(A0 to A23)
(D0 to D7)
(D8 to D15)
φ1
At
quadruple
consecutive
access
At
double
consecutive
access
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 8016, 8216, 8416, 8616).
2: The burst ROM access can be selected only in the case of in Figure 18.