CHAPTER 14: External Bus Interface
790 FM4 Peripheral Manual, Doc. No. 002-04856 Rev. *E
Read Access
To improve the efficiency of the read access of the external bus interface, the preceding read of the next
read access is provided.
If preceding access is enabled, it will improve read access efficiency because, when external bus
interface read access is performed continuously, the internal bus requests received while the internal bus
response is WAIT are read to the external device and data is preceded without waiting for internal bus
response OK.
At the setting where the preceding access is available, the preceding access of which transfer is not
cancelled may be executed to the external bus interface. In such case, the read value is disposed of and
it does not affect the CPU operation.
For the operation example of preceding access, see Figure 3-27. The read access to the external
memory is executed before accepting the second read request from the internal bus and after the
accepting the request, the read data is returned to the CPU. At the third time, the preceding access is
executed in the same way, the read data is disposed of because the read request is cancelled at
accepting the access request.
However, when the above mentioned preceding access is unavailable due to the specifications of the
external device, set the preceding function ineffective with register settings (AMODE:WAEN).
Figure 3-27 Operation Example of Preceding Read Access Occurred
Read(B)
Read(C)
Read(A)
A
A
B
B C
Internal bus request
Internal bus read data
Internal bus response
OK WAIT OK WAIT
OK
External bus interface
(MCSX)
External bus interface
(MOEX)
External bus interface
(MADATA)
Discard read data
1st time 2nd time 3rd time → Cancel read request
■Prefetch Enabled Read (Operation Example)
Read(A) Read(B) Read(C)
Internal bus request retiming
(AHB → external bus)
■Prefetch Disabled Read (Operation Example)
Read(B)
Read(C)
Read(A)
A
B
B
OK WAIT OK WAIT
OK
Read(A)
Read(B)
Prefetch Prefetch
A
1st time 2nd time 3rd time → Cancel read request
Internal bus request
Internal bus read data
Internal bus response
External bus interface
(MCSX)
External bus interface
(MOEX)
External bus interface
(MADATA)
Internal bus request retiming
(AHB → external bus)
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