Datasheet

SAM9263 [DATASHEET]
Atmel-6249N-ATARM-SAM9263-Datasheet_14-Mar-16
150
19.4 Arbitration
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 to the user the possibility to choose between 2 arbitration types, and this for each slave:
1. Round-Robin Arbitration (the default)
2. Fixed Priority Arbitration
This choice is given through the field ARBT of the Slave Configuration Registers (MATRIX_SCFG).
Each algorithm may be complemented by selecting a default master configuration for each slave.
When a re-arbitration has to be done, it is realized only under some specific conditions detailed in the following
paragraph.
19.4.1 Arbitration rules
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, arbitration may only take place during
the following cycles:
1. Idle Cycles: when a slave is not connected to any master or is connected to a master which is not currently
accessing it.
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 defined length burst,
predicted end of burst matches the size of the transfer but is managed differently for undefined length burst.
(See Section 19.4.1.1 “Undefined Length Burst Arbitration”.)
4. Slot Cycle Limit: when the slot cycle counter has reach the limit value indicating that the current master
access is too long and must be broken.(See Section 19.4.1.2 “Slot Cycle Limit Arbitration”.)
19.4.1.1Undefined Length Burst Arbitration
In order to avoid too long slave handling during undefined length bursts (INCR), the Bus Matrix provides specific
logic in order to re-arbitrate before the end of the INCR transfer.
A predicted end of burst is used as for defined length burst transfer, which is selected between the following:
1. Infinite: no predicted end of burst is generated and therefore INCR burst transfer will never be broken.
2. Four beat bursts: predicted end of burst is generated at the end of each four beat boundary inside INCR
transfer.
3. Eight beat bursts: predicted end of burst is generated at the end of each eight beat boundary inside INCR
transfer.
4. Sixteen beat bursts: predicted end of burst is generated at the end of each sixteen beat boundary inside
INCR transfer.
This selection can be done through the field ULBT of the Master Configuration Registers (MATRIX_MCFG).
19.4.1.2 Slot Cycle Limit Arbitration
The Bus Matrix contains specific logic to break too long accesses such as very long bursts on a very slow slave
(e.g. an external low speed memory). At the beginning of the burst access, a counter is loaded with the value
previously written in the SLOT_CYCLE field of the related Slave Configuration Register (MATRIX_SCFG) and
decreased at each clock cycle. When the counter reaches zero, the arbiter has the ability to re-arbitrate at the end
of the current byte, half word or word transfer.