Power and clock management
RM0082
816/844
Doc ID 018672 Rev 1
37
Power and clock management
37.1 Overview
Power consumption is an important design aspect of any modern system. Power
management techniques allows to reduce power consumption ensuring requested
performance by utilization.
37.1.1 Power
management
techniques
System Control State Machine
is a device feature designed to support reduction of power
consumption controlling clock inputs to the CPU. It presents four state:
●
SLEEP
●
DOZE (reset state)
●
SLOW
●
NORMAL
All transactions between states are software controllable except for SLEEP to DOZE that is
activated only by a hardware event.
The following items describe the power management techniques:
●
Dynamic Frequency Scaling
applicable in NORMAL state
This technique uses dynamic selection of the optimal frequency to allow a task to be
performed in the required amount of time.
As described in
Chapter 11: Clock & reset system
chapter PLL1 (Sys) provide frequency to
the system. By default also DRAM is driven by PLL1, this mode is called
synchronous
DRAM.
It is possible to use PLL2 to drive DRAM, this mode is called
asynchronous
DRAM.
In asynchronous DRAM mode it is possible to change system frequency (PLL1) without
affecting DRAM works, for this reason to apply Dynamic Frequency Scaling System has to
work in asynchronous DRAM mode.
Frequency changes are applied with small delay, due to PLL lock time (see formula below to
calculate this delay)
●
Dynamic Clock Switching
It is possible to dynamically switch off, or on, the clock to modulate group in accordance with
their use.
This operation is performed without delay.
●
Combining Frequency Scalling and Clock techniques
In NORMAL state the best active power saving is obtained by combining the power
management techniques previously described.
These techniques is aimed at reducing power consumption of a device allowing a fast
response to critical task (that can be performed always at maximum frequency, if needed).
●
Statically Frequency Selection and Clock Switching OFF
In a well known system it is possible to define statically, based on performances defined in
the manufacturing process, the operating frequency and the group that do not need clock.