Functional Description
ARM DDI 0500D
Copyright © 2013-2014 ARM. All rights reserved.
2-9
ID021414
Non-Confidential
2.3
Clocking and resets
The following sections describe clocking and resets:
•
Clocks
.
•
Input synchronization
on page 2-13
.
•
Resets
on page 2-14
.
2.3.1
Clocks
The Cortex-A53 processor has a single clock input,
CLKIN
. All cores in the Cortex-A53
processor and the SCU are clocked with a distributed version of
CLKIN
.
The Cortex-A53 processor has the following clock enable signals:
•
PCLKENDBG
.
•
ACLKENM
on page 2-10
.
•
ACLKENS
on page 2-10
.
•
SCLKEN
on page 2-11
.
•
ATCLKEN
on page 2-12
.
•
CNTCLKEN
on page 2-12
.
PCLKENDBG
The processor includes an APB interface to access the debug and performance monitoring
registers. Internally this interface is driven from
CLKIN
. A separate enable signal,
PCLKENDBG
, is provided to enable the external APB bus to be driven at a lower frequency,
that must be an integer ratio of
CLKIN
. If the debug infrastructure in the system is required to
be fully asynchronous to the processor clock, you can use a synchronizing component to
connect the external AMBA APB to the processor.
Figure 2-2
shows a timing example of
PCLKENDBG
that changes the
CLKIN
to
PCLK
frequency ratio from 3:1 to 1:1.
Figure 2-2 PCLKENDBG with CLKIN:PCLK ratio changing from 3:1 to 1:1
Note
Figure 2-2
shows the timing relationship between the debug APB clock,
PCLK
and
PCLKENDBG
, where
PCLKENDBG
asserts one clock cycle before the rising edge of
PCLK
.
It is important that the relationship between
PCLK
and
PCLKENDBG
is maintained.
PCLK
PCLKENDBG
asserts one
CLKIN
cycle before the rising edge of
PCLK
1
CLKIN
cycle
CLKIN
:
PCLK
= 3:1
CLKIN
:
PCLK
= 1:1
PCLKENDBG
CLKIN
1
CLKIN
cycle