Power and clock management
RM0082
820/844
Doc ID 018672 Rev 1
37.3
Dynamic frequency scaling
Dynamic Frequency Scaling (DFS) is generally used when the work-load is not CPU-bound.
It reduces processor’s instructions in a given amount of time, thus reducing performance but
also consumption. It is very efficient to run briefly at peak speed and at a reduced clock rate
for a long time.
It is possible to change PLL frequency in NORMAL state, but it generates undesirable
frequency overshoot/undershoot. To avoid this it is better to switch in Slow state, change
PLL frequency, disable Dithering (if enabled), wait for PLLlock signal, switch again in Normal
mode and enable again Dithering (if it was originally enabled).
With the following formula it is possible to calculate delay time introduced by PLL for
frequency changes.
Lock time = 4ms/(decimal equivalent of PLL Charge Pump bit 1)
PLL Charge Pump bits are PLL1_CTR.CP
So, for example with CP = 01110 = 14 (decimal)
Lock time = 4ms/15 = 267 us
There are two way to wait PLL stabilization, software and hardware, look at section
for details.
37.4 Dynamic
clock
switching
Like DFS, Dynamic Clock Switching (DCS) is a power-management technique aimed at
reducing active power consumption of a device, whereas DFS change frequency of all
modules using CLK_Pll1 signal. DCS could switch OFF completely the clock of an unused
modules and quickly switch ON when its use is required.
With this technique the processor, or system, could run at maximum frequency maintained
highest performances.
To have maximum flexibility DCS is fully software controlled through
Chapter 13: Miscellaneous registers (Misc)
DCS is useful when a real-time application is waiting for an event. The system can switch
OFF clock of modules not used and enable them, with a low latency, when needed.
Modules that support this feature are shown in
Table 738.
Techniques applicable in NORMAL state
Technique
Synchronous DRAM
Asynchronous DRAM
Dynamic Frequency Scalling (DFS)
Denied
Allowed
Dynamic Clock Switching (DCS)
Allowed
Allowed
Combining DFC+DCS
Denied
Allowed
Statically Frequency Selection and Clock
Switching OFF
Allowed
Allowed