Hardware Reference
In-Depth Information
Voltage and Current Control Framework
The
Voltage and Current Control Framework
(
V/I Framework
)[
5
] is a quite specific
support focusing on the efficiency of voltage regulators. In the architecture explo-
ration, we highlighted how modern SoC architectures are composed of multiple
voltage domains to fit specific requirements of each hardware block. In general, the
voltage domains within a SoC could have some dependency relation between them.
Sometimes these voltage domains are directly controlled by a dedicated voltage
regulator usually provided by an external companion chip.
Each device in the system is powered by a certain voltage domain and, according
to the specific functionalities required by a device, the current drained from the
domain could also be very different. For instance, if we consider an audio-codec
controller, its current drain is very different if we are listening to some audio stream
via a loudspeaker or we are simply performing some digital audio mixing activities.
A physics study of the dynamics of a regulator device shows that its efficiency is
highly affected by the instantaneous current load. The
Regulator Power Efficiency
(RPE)
of a regulator is defined in Eq.
6.1
.
RPE
=
P
out
/P
in
(6.1)
Equation
6.1
compares the amount of power
P
in
that is presented as input to the
regulator, and how much
P
out
we are able to derive from it; it is a direct measure of
how much energy is lost in the regulator itself.
It is known that when the regulator works in normal mode, it is able to efficiently
support only current loads over a certain threshold value. On the contrary, once the
current load on the corresponding voltage domain drops under this threshold, the
current requirement could be satisfied with a better efficiency only switching the
regulator to an idle operating mode. This kind of behavior of voltage regulators are
worth to be considered in order to implement a really holistic approach to power
management in a modern embedded system. The framework presented in this para-
graph has been introduced in the Linux framework quite recently, but provides a well
designed and mature support to simplify the exploitation of this kind of optimization.
The framework is composed of four separate interfaces:
regulator
, allows a regulator driver to register a set of required operations to the
core framework;
consumer
, allows a device to notify voltage and current requirements to the
regulator driver;
platform
, allows the system platform code to define the voltage domains, their
dependencies and thus the creation of the regulator tree;
userspace,
exports a lot of useful voltage/current data and operation mode statis-
tics via a
sysfs
interface to support device power consumption and status
monitoring.
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