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25.3
Memory Mapping
The Bus Matrix provides one decoder for every AHB master interface. The decoder offers each
AHB master several memory mappings. Each memory area may be assigned to several slaves.
Booting at the same address while using different AHB slaves (i.e. external RAM, internal ROM
or internal Flash, etc.) becomes possible.
The Bus Matrix user interface provides the Master Remap Control Register (MATRIX_MRCR),
that performs remap action for every master independently.
25.4
Special Bus Granting Mechanism
The Bus Matrix provides some speculative bus granting techniques in order to anticipate access
requests from masters. This mechanism reduces latency at first access of a burst, or single
transfer, as long as the slave is free from any other master access, but does not provide any
benefit as soon as the slave is continuously accessed by more than one master, since arbitra-
tion is pipelined and has no negative effect on the slave bandwidth or access latency.
This bus granting mechanism sets a different default master for every slave.
At the end of the current access, if no other request is pending, the slave remains connected to
its associated default master. A slave can be associated with three kinds of default masters:
• no default master
• last access master
• fixed default master
To change from one type of default master to another, the Bus Matrix user interface provides the
Slave Configuration Registers, one for every slave, that set a default master for each slave. The
Slave Configuration Register contains two fields: DEFMSTR_TYPE and FIXED_DEFMSTR. The
2-bit DEFMSTR_TYPE field selects the default master type (no default, last access master, fixed
default master), whereas the 4-bit FIXED_DEFMSTR field selects a fixed default master pro-
vided that DEFMSTR_TYPE is set to fixed default master. Refer to
Slave Configuration Registers”
25.4.1
No Default Master
After the end of the current access, if no other request is pending, the slave is disconnected from
all masters.
This configuration incurs one latency clock cycle for the first access of a burst after bus Idle.
Arbitration without default master may be used for masters that perform significant bursts or sev-
eral transfers with no Idle in between, or if the slave bus bandwidth is widely used by one or
more masters.
This configuration provides no benefit on access latency or bandwidth when reaching maximum
slave bus throughput, irregardless of the number of requesting masters.
25.4.2
Last Access Master
After the end of the current access, if no other request is pending, the slave remains connected
to the last master that performed an access request.
This allows the Bus Matrix to remove the one latency cycle for the last master that accessed the
slave. Other non-privileged masters still get one latency clock cycle if they want to access the
same slave. This technique is useful for masters that mainly perform single accesses or short
bursts with some Idle cycles in between.
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