Functional Description
ARM DDI 0388I
Copyright © 2008-2012 ARM. All rights reserved.
2-7
ID073015
Non-Confidential
Reset modes
The reset signals present in the Cortex-A9 design enable you to reset different parts of the
processor independently.
shows the reset signals, and the combinations and possible
applications that you can use them in.
Power-on reset
You must apply power-on or
cold
reset to the Cortex-A9 uniprocessor when power is first
applied to the system. In the case of power-on reset, the leading edge, that is the falling edge, of
the reset signals do not have to be synchronous to
CLK
, but the rising edge must be.
You must assert the reset signals for at least nine
CLK
cycles to ensure correct reset behavior.
ARM recommends the following reset sequence:
1.
Apply
nCPURESET
and
nDBGRESET
, plus
nNEONRESET
if the SIMD MPE is
present.
2.
Wait for at least nine
CLK
cycles, plus at least one cycle in each other clock domain, or
more if the documentation for other components requires it. There is no harm in applying
more clock cycles than this, and maximum redundancy can be achieved by applying 15
cycles on every clock domain.
3.
Stop the
CLK
clock input to the Cortex-A9 uniprocessor. If there is a data engine present,
use
NEONCLKOFF
. See
4.
Wait for the equivalent of approximately 10 cycles, depending on your implementation.
This compensates for clock and reset tree latencies.
5.
Release all resets.
6.
Wait for the equivalent of another approximately 10 cycles, again to compensate for clock
and reset tree latencies.
7.
Restart the clock.
Software reset
A processor or warm reset initializes the majority of the Cortex-A9 processor, apart from its
debug logic. Breakpoints and watchpoints are retained during a processor reset. Processor reset
is typically used for resetting a system that has been operating for some time. Use the same reset
sequence described in
with the only difference that
nDBGRESET
must remain
HIGH during the sequence, to ensure that all values in the debug registers are maintained.
Table 2-1 Reset modes
Mode
nCPURESET
nNEONRESET
nDBGRESET
Power-on reset, cold reset
0
0
0
Processor reset, soft or warm reset
0
0
1
SIMD MPE power-on reset
1
0
1
Debug logic reset
1
1
0
No reset, normal run mode
1
1
1