Functional Description
ARM DDI 0500D
Copyright © 2013-2014 ARM. All rights reserved.
2-12
ID021414
Non-Confidential
•
If there are any physical effects that could occur while changing the clock frequency,
ARM recommends that the clock ratio is changed only while the
STANDBYWFIL2
output of the processor is asserted.
•
The input signal
SCLKEN
exists in the Cortex-A53 processor if it is configured to
include the CHI interface.
ATCLKEN
The ATB interface is a synchronous interface that can operate at any integer multiple that is
equal to or slower than the main processor clock,
CLKIN
, using the
ATCLKEN
signal. For
example, the
CLKIN
to
ATCLK
frequency ratio can be 1:1, 2:1, or 3:1, where
ATCLK
is the
ATB bus clock.
ATCLKEN
asserts one
CLKIN
cycle before the rising edge of
ATCLK
. If you
change the
CLKIN
to
ATCLK
frequency ratio, you must change
ATCLKEN
correspondingly.
Figure 2-6
shows a timing example of
ATCLKEN
that changes the
CLKIN
to
ATCLK
frequency ratio from 3:1 to 1:1.
Figure 2-6 ATCLKEN with CLKIN:ATCLK ratio changing from 3:1 to 1:1
Note
Figure 2-6
shows the timing relationship between the ATB clock,
ATCLK
and
ATCLKENDBG
, where
ATCLKENDBG
asserts one clock cycle before the rising edge of
ATCLK
. It is important that the relationship between
ATCLK
and
ATCLKENDBG
is
maintained.
CNTCLKEN
The
CNTVALUEB
is a synchronous 64-bit binary encoded counter value that can operate at
any integer multiple that is equal to or slower than the main processor clock,
CLKIN
, using the
CNTCLKEN
signal. For example, you can set the
CLKIN
to
CNTCLK
frequency ratio to 1:1,
2:1, or 3:1, where
CNTCLK
is the system counter clock.
CNTCLKEN
asserts one
CLKIN
cycle prior to the rising edge of
CNTCLK
.
Figure 2-7 on page 2-13
shows a timing example of
CNTCLKEN
that changes the
CLKIN
to
CNTCLK
frequency ratio from 3:1 to 1:1.
ATCLK
ATCLKEN
asserts one
CLKIN
cycle before the rising edge of
ATCLK
1
CLKIN
cycle
CLKIN
:
ATCLK
= 3:1
CLKIN
:
ATCLK
= 1:1
ATCLKEN
CLKIN
1
CLKIN
cycle