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Linux Power Optimization Features
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SPRAC74A–February 2017–Revised March 2017
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Copyright © 2017, Texas Instruments Incorporated
AM335x Low Power Design Guide
3.3.2.1 Shunt Resistors
Measuring voltage across shunt resistors is relatively straightforward, it is important that the multimeter
used has enough resolution and range to capture meaningful voltage readings, typically expect tenths of
milivolts to single digit milivolt range.
Since typically using shunts involve some aspect of manual measurement, be sure that the power
measured actually corresponds to the desired state or behavior. Checking this can be simple as console
output, or using a GPIO or LED toggle to indicate different modes.
3.3.2.2 INA Devices
Since INAs are I2C devices, it is possible to access the devices directly over the I2C bus. There are also
several software tools created with this functionality. A simple bash script for reading the INA226, and the
open source powertool will be discussed. Depending on the hardware setup, an external I2C master may
or may not be required. For power consumption measurement when SOC is inactive, an external device is
required.
3.3.2.2.1 Powertool
Powertool is an open source tool for reading INA226 devices on an I2C bus. The source code for
Powertool can be found on github. Powertool requires the FTDI USB to I2C adapter from Spectrum
Digital, or one can use powertool directly from a device connected to the I2C bus with the relevant INA
devices. On the AM335x EVM, the INA devices are on an external I2C bus, and therefore require an
external device, such as a BeagleBone Black or the FTDI USB I2C adapter to sample power data.
Powertool uses configuration files to define measured rails, several Texas Instruments EVMs are
supported out of the box, including the AM335x GP EVM. For specific syntax and build requirements, see
the powertool documentation.
3.3.2.2.2 Manual I2C Data Collection
An example shell script is included in the Appendix A to demonstrate that, if desired, INA devices can be
read directly from the I2C bus.
3.3.3 Confirm and Validate Hardware Changes
As changes are made to the default configuration, verification should be done to ensure that the changes
had the desired effect without any unintended consequences. This section highlights two tools used to
read the power and clock state of the underlying hardware. The included Power Reset Clock Management
register dump script comes with set of tools for interpreting the output. Omapconf is another utility that
provides more detailed information and can be used to verify the state of modules on the AM335x SOC.
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