SAM3X8C Atmel Corporation, SAM3X8C Datasheet - Page 333

• Manufacturer Part Number: SAM3X8C
• Manufacturer: Atmel Corporation

22.4.2
22.4.3
22.5
22.5.1
11057A–ATARM–17-Feb-12
11057A–ATARM–17-Feb-12
Arbitration
Last Access Master
Fixed Default Master
Arbitration Rules
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.
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
FIXED_DEFMSTR field allows to choose a fixed default master provided that DEFMSTR_TYPE
is set to fixed default master. Please refer to the Bus Matrix user interface description.
The Bus Matrix provides an arbitration mechanism that allows to reduce latency when conflict
cases occur, basically when two or more masters try to access the same slave at the same time.
One arbiter per AHB slave is provided, allowing to arbitrate each slave differently.
The Bus Matrix provides the user the possibility to choose between 2 arbitration types for each
slave:
This choice is given through the ARBT field of the Slave Configuration Registers
(MATRIX_SCFG).
Each algorithm may be complemented by selecting a default master configuration for each
slave.
When re-arbitration has to be done, it is realized only under specific conditions as detailed in the
following paragraph.
Each arbiter has the ability to arbitrate between two or more different master’s requests. In order
to avoid burst breaking and also to provide the maximum throughput for slave interfaces, arbitra-
tion may only take place during the following cycles:
1. Round-Robin Arbitration (the default)
2. Fixed Priority Arbitration
1. Idle Cycles: when a slave is not connected to any master or is connected to a master
2. Single Cycles: when a slave is currently doing a single access.
3. End of Burst Cycles: when the current cycle is the last cycle of a burst transfer. For
4. Slot Cycle Limit: when the slot cycle counter has reached the limit value indicating that
which is not currently accessing it.
defined length burst, predicted end of burst matches the size of the transfer but is man-
aged differently for undefined length burst (See
Burst Arbitration” on page
the current master access is too long and must be broken (See
Cycle Limit Arbitration” on page
334“).
334).
Section 22.5.1.1 “Undefined Length
Section 22.5.1.2 “Slot
SAM3X/A
SAM3X/A
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