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Optimising software for power efficiency (Part 1)

Posted: 17 Jun 2014     Print Version  Bookmark and Share

Keywords:power consumption  algorithmic  hardware  data flow  static 

In a real system, a device will rarely if ever draw the worst-case power, as applications do not use all the processing elements, memory, and I/O for long periods of time, if at all. In general, a device provides many different I/O peripherals, though only a portion of them are needed, and the device cores may only need to perform heavy computation for small portions of time, accessing just a portion of memory. Typical power consumption then may be based on the assumed "general use case" example application that may use anywhere from 50% to 70% of the processor's available hardware components at a time. This is a major aspect of software applications that we are going to be taking advantage of in order to optimise power consumption.

Measuring power consumption
Measuring power is hardware dependent: some embedded processors provide internal measurement capabilities; processor manufacturers may also provide "power calculators" which give some power information; there are a number of power supply controller ICs which provide different forms of power measurement capabilities; some power supply controllers called VRMs (voltage regulator modules) have these capabilities internal to them to be read over peripheral interfaces; and finally, there is the old-fashioned method of connecting an ammeter in series to the core power supply.

Measuring power using an ammeter. The "old-fashioned" method is to measure power via the use of an external power supply connected in series to the positive terminal of an ammeter, which connects via the negative connector to the DSP device power input, as shown in figure 1.

Figure 1: Measuring power via ammeters.

Note that there are three different set-ups shown in figure 1, which are all for a single processor. This is due to the fact that processor power input is isolated, generally between cores (possibly multiple supplies), peripherals, and memories. This is done by design in hardware as different components of a device have different voltage requirements, and this is useful to isolate (and eventually optimise) the power profile of individual components.

In order to properly measure power consumption, the power to each component must be properly isolated, which in some cases may require board modification, specific jumper settings, etc. The most ideal situation is to be able to connect the external supply/ammeter combination as close as possible to the processor power input pins.

Alternatively, one may measure the voltage drop across a (shunt) resister which is in series with the power supply and the processor power pins. By measuring the voltage drop across the resistor, current is found simply by calculating I = V/R.

Measuring power on a half sensor IC. In order to simplify efficient power measurement, many embedded vendors are building boards that use a Hall-effect-based sensor. When a Hall sensor is placed on a board in the current path to the device's power supply, it generates a voltage equivalent to the current times some coefficient with an offset.

In the case of Freescale's MSC8144 DSP Application Development System board, an Allegro ACS0704 Hall sensor is provided on the board, which enables such measurement. With this board, the user can simply place a scope to the board, and view the voltage signal over time, and use this to calculate average power using Allegro's current to voltage graph, shown in figure 2.

Using figure 2, we can calculate input current to a device based on measuring potential across Vout as:

I = (Vout—2:5)*10A

Using VRM ICs. Finally, some voltage regulator module power supply ICs (VRMs) are used to split a large input voltage into a number of smaller ones to supply individual sources at varying potentials, measure current/power consumption and store the values in registers to be read by the user. Measuring current via the VRM requires no equipment, but this sometimes comes at the cost of accuracy and real-time measurement.

Figure 2: Hall effect IC voltage-to-current graph ( ./0704/ 0704-015.pdf).

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