AT32AP7001_1 Datasheet, PDF (141/829 Pages) ATMEL Corporation – AVR32 32-bit Microcontroller

AT32AP7001
16.4.3.1
When a re-arbitration must be done, specific conditions apply. See Section 16.4.3.1 ”Arbitration
Rules” on page 141.
Arbitration Rules
Each arbiter has the ability to arbitrate between two or more different master 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. 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 man-
aged differently for undefined length burst. See Section “•” on page 141.
4. Slot Cycle Limit: When the slot cycle counter has reached the limit value indicating that
the current master access is too long and must be broken. See Section “•” on page 141.
• Undefined Length Burst Arbitration
In order to avoid long slave handling during undefined length bursts (INCR), the Bus Matrix pro-
vides specific logic in order to re-arbitrate before the end of the INCR transfer. A predicted end
of burst is used as a defined length burst transfer and can be selected from among the following
five possibilities:
1. Infinite: No predicted end of burst is generated and therefore INCR burst transfer will
never be broken.
2. One beat bursts: Predicted end of burst is generated at each single transfer inside the
INCP transfer.
3. Four beat bursts: Predicted end of burst is generated at the end of each four beat bound-
ary inside INCR transfer.
4. Eight beat bursts: Predicted end of burst is generated at the end of each eight beat
boundary inside INCR transfer.
5. 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
(MCFG).
• Slot Cycle Limit Arbitration
The Bus Matrix contains specific logic to break 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 (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.
32015G–AVR32–09/09
141