Datasheet

308
11011B–ATARM–21-Feb-12
SAM3N
21.2.3 Master to Slave Access
All the Masters can normally access all the Slaves. However, some paths do not make sense,
for example allowing access from the Cortex-M3 S Bus to the Internal ROM. Thus, these paths
are forbidden or simply not wired and shown as “-” in the following table.
21.3 Memory Mapping
Bus Matrix provides one decoder for every AHB Master Interface. The decoder offers each AHB
Master several memory mappings. In fact, depending on the product, each memory area may be
assigned to several slaves. Booting at the same address while using different AHB slaves (i.e.
internal ROM or internal Flash) becomes possible.
21.4 Special Bus Granting Techniques
The Bus Matrix provides some speculative bus granting techniques in order to anticipate access
requests from some masters. This mechanism allows to reduce latency at first accesses of a
burst or single transfer. The bus granting mechanism allows to set a 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 and fixed default master.
21.4.1 No Default Master
At the end of the current access, if no other request is pending, the slave is disconnected from
all masters. No Default Master suits low power mode.
21.4.2 Last Access Master
At 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.
21.4.3 Fixed Default Master
At the end of the current access, if no other request is pending, the slave connects to its fixed
default master. Unlike last access master, the fixed master doesn’t change unless the user mod-
ifies it by a software action (field FIXED_DEFMSTR of the related MATRIX_SCFG).
To change from one kind of default master to another, the Bus Matrix user interface provides the
Slave Configuration Registers, one for each slave, that allow to 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 allows to choose
the default master type (no default, last access master, fixed default master) whereas the 4-bit
Table 21-2. SAM3N Master to Slave Access
Masters 0 1 2
Slaves
Cortex-M3 I/D
Bus
Cortex-M3 S
Bus
PDC
0 Internal SRAM - X X
1 Internal ROM X - X
2 Internal Flash X - -
3 Peripheral Bridge - X X