CONNECTION WITH EXTERNAL DEVICES
7902 Group User’s Manual
3-25
3.2 Chip select wait controller
(2) Burst ROM access
When ROM, etc., supporting burst access, is allocated to area CSi, the burst access for the maximum
of 8 bytes becomes available if the burst ROM access is specified (the burst ROM access select bit
= “1”). The burst ROM access is valid only when the external data bus width = 16 bits with instructions
prefetched. In the other cases, normal access is specified regardless of the burst ROM access select
bit.
Figure 3.2.13 shows the operating waveform at burst ROM access.
Also, for the instruction prefetch, refer to section “2.2.1 Instruction prefetch.”
At instruction prefetch with burst ROM access, 8 bytes are fetched from an 8-byte boundary. (See
Figure 3.2.13 (a): quadruple consecutive access.)
At branch, 4 bytes are fetched from a 4-byte boundary regardless of the low-order 2 bits (A1, A0) of
the branch destination address. (See Figure 3.2.13 (b): double consecutive access.) In this case, the
number of fetched bytes depends on the branch destination address. (See Table 2.2.3.)
Also, the address of data (instruction) to be fetched next controls the following operations as below:
If this address is placed at an 8-byte boundary, data (instruction) will be fetched in a unit of 8 bytes.
(See Figure 3.2.13 (a): quadruple consecutive access.)
If this address is placed at a 4-byte boundary, after 4-byte data is fetched (See Figure 3.2.13 (b):
double consecutive access.), data will be fetched in a unit of 8 bytes. (See Figure 3.2.13 (a):
quadruple consecutive access.)
Fig. 3.2.13 Operating waveform at burst ROM access
(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
Note: The above is applied when 1 bus cycle = 1
φ + 1φ. For details of the bus cycle
types, refer to section “(1) Bus cycle.”