Level 2 Memory System
ARM DDI 0500D
Copyright © 2013-2014 ARM. All rights reserved.
7-11
ID021414
Non-Confidential
7.3.4
Read response
The ACE master can delay accepting a read data channel transfer by holding
RREADY
LOW
for an indeterminate number of cycles.
RREADY
can be deasserted LOW between read data
channel transfers that form part of the same transaction.
The ACE master asserts the read acknowledge signal
RACK
HIGH in the
ACLK
cycle
following acceptance of the last read data channel transfer for a transaction.
RACK
is asserted
in AXI3 compatibility mode in addition to ACE configurations.
Note
•
For interoperability of system components, ARM recommends that components
interfacing with the ACE master are fully ACE compliant with no reliance on the subset
of permitted
RACK
behavior described for the Cortex-A53 processor.
•
If the interconnect does not perform hazarding between coherent and non-coherent
requests, then, after it has returned the first transfer of read data for a non-coherent read,
it must return all the remaining read transfers in the transaction, without requiring
progress of any snoops to the cluster that could be to the same address.
7.3.5
Write response
The ACE master requires that the slave does not return a write response until it has received the
write address.
The ACE master always accepts write responses without delay by holding
BREADY
HIGH.
The ACE master asserts the write acknowledge signal
WACK
HIGH in the
ACLK
cycle
following acceptance of a write response.
WACK
is asserted in AXI3 compatibility mode in
addition to ACE configurations.
Note
For interoperability of system components, ARM recommends that components interfacing
with the ACE master are fully ACE compliant with no reliance on the subset of permitted
BREADY
and
WACK
behavior described for the Cortex-A53 processor.
7.3.6
Barriers
The Cortex-A53 processor supports sending barrier transactions to the interconnect, or
terminating barriers within the cluster.
•
To send barriers on the ACE interface, set
SYSBARDISABLE
to LOW.
•
To terminate barriers within the cluster, set
SYSBARDISABLE
to HIGH.
If you terminate barriers within the cluster, ensure that your interconnect and any peripherals
connected to it do not return a write response for a transaction until that transaction would be
considered complete by a later barrier. This means that the write must be observable to all other
masters in the system. ARM expects the majority of peripherals to meet this requirement.
For best performance, ARM recommends that barriers are terminated within the cluster.
7.3.7
AXI3 compatibility mode
The Cortex-A53 processor implements an AXI3 compatibility mode that enables you to use the
processor in a standalone environment where the AMBA 4 ACE interface is not required and
the processor does not propagate barriers outside of the cluster. To enable this mode you must