Texas Instruments TMS320 User Manual page 158

Power Management
4.6.5 Scaling Voltage and Frequency
4-54
Resource tracking is intended to be used in conjunction with PWRM's device
initialization feature:
1) At boot time PWRM initializes resources to be powered down.
2) When the application needs to use a resource (for example, DMA) it calls
PWRM_setDependency to register the dependency. PWRM then
automatically powers up the resource (for example, by un-idling the DMA
clock domain).
3) When the resource is no longer needed, the application calls
PWRM_releaseDependency. If there are no other dependencies still
registered on that resource, PWRM automatically powers it down (for
example, by idling the DMA domain).
You may have legacy code that cannot be easily modified to add PWRM
resource tracking calls. For example, a driver may only be available in binary
form. In such situations, you can add calls to PWRM_setDependency at boot
time to declare dependencies in legacy code.
The resources tracked by PWRM will vary from device to device, and are
described in the corresponding DSP/BIOS release notes.
Active power dissipation of a CMOS-based DSP is linearly proportional to the
clock rate (frequency), and quadratically proportional to the operating
voltage. Additionally, the operating voltage determines the maximum clock
rate available.
Therefore, if an application can reduce the CPU clock rate and still meet its
processing deadlines, it can produce a linearly proportional savings in power
dissipation. However, reducing the CPU clock rate also proportionally
extends the execution time, so the application must be carefully analyzed to
make sure it can still meets its real-time requirements.
If the clock frequency can be reduced and the new frequency is compatible
with a lower operating voltage supported by the DSP, then potentially
significant additional savings can be also be made by reducing the voltage,
due to the quadratic relationship.
The PWRM module allows an application to call PWRM_changeSetpoint to
change the operating voltage and frequency (V/F). So, for example, when an
application switches to a mode with reduced processing requirements, it can
step down the voltage and frequency to reduce power usage. Or, an
application might accumulate "slack" time in its data-dependent processing,
and then reduce V/F to absorb the slack time while running at lower power.