With the rapid growth of low-power and battery-operated embedded systems in the consumer and industrial markets, the energy consumed by systems using embedded processors has become a core attribute – it is just as important as clock speeds and system performance. Designs must be smaller than ever with superior system performance, and must incorporate advanced power management for prolonged battery life and better thermal management to reduce reliance on components such as heat sinks and cooling fans. Optimum power efficiency is not only applicable to mobile devices but also to wired devices due to consumers’ new focus on power efficiency. Despite the importance of balancing performance and energy in an embedded system, surprisingly, very few processor boards are available with a mechanism to measure these parameters. What’s even more surprising is the relative ease in which power/energy measurement capabilities can be implemented.
Maximum Power versus Typical Power
During the design process, it is important to ensure that an embedded system operates within the specifications of its components. As a result, it is also important to determine the maximum power consumption that a system uses under certain conditions. However, once in the hands of the end user, maximum power consumption may not be as important. Typical power becomes a much more relevant metric to represent the power that is consumed during normal, day-to-day operation of an embedded system, and it has the most impact on whether that system consumes power efficiently. Furthermore, you can easily convert typical power measurements into an energy factor that directly relates to battery life, cost of operation, and heat dispersion.
Methods for Measuring Current
A large percentage of the energy consumed in an embedded system can be attributed to the current draw of the system's microprocessor. Therefore, you can use accurate measurements of the amount of current that the microprocessor sinks under various operating conditions to determine the power efficiency. Current measurement is typically done with a shunt resistor placed in series with the current flow in a circuit. This resistor is specifically chosen to be high-precision and low-impedance so as not to interfere greatly with the circuit being monitored. Because the value of the resistor is known, by measuring the voltage drop across the shunt resistor, you can accurately calculate the current using Ohm’s law. Another method for measuring current, although less common, is to use expensive current probes, in which case you do not need a shunt resistor.
High-Side Current Measurements
There are two different configurations that you can use when measuring current with a series shunt resistor. The first, referred to as high-side current measurement, involves placement of the shunt resistor in series with the supply path of the load circuit as shown in Figure 1.
Figure 1. High-Side Current Measurement
The primary advantage of using this method for current measurements is that you can isolate the current draw for a specific component of the overall system with minimal losses. Specifically, there is not a risk of current leakage into other components of the system. The main disadvantage of this method is that in some applications, a high common-mode voltage may be present on both sides of the shunt resistor. This high common-mode voltage could potentially damage the device you are using to measure the differential voltage across the shunt.
National Instruments offers several digital multimeters (DMMs) with high-input voltage protection. For example, the NI PCI-4070 DMM has 300 VDC input protection with 6½ digits of precision. You can use the PCI-4070 to acquire current by probing specific points of interests in a circuit or an embedded system. As you acquire the data, you can use the NI LabVIEW graphical programming environment to process and analyze current measurements. By using NI hardware and software products to custom design your own applications, you can accurately benchmark the power requirements of embedded systems.
Low-Side Current Measurements
You also can use low-side current measurement to measure the current. In this configuration, you place a shunt resistor in series with the return path of the load as shown in Figure 2.
Figure 2. Low-Side Current Measurement
While low-side current measurement does remove the possibility of affecting high common-mode voltage levels, this method does not come without disadvantages. Only current that is directly returned to the supply is measured, creating errors in measurement for loads that leak current to other grounds such as a chassis ground or control circuitry grounds.
To measure low-side currents with the perfect balance of cost and functionality, consider the NI PCI-6251 multifunction data acquisition board for its high-sampling rates of 1 MS/s (multichannel), 16-bit measurement resolution, and flexible input and output capabilities. You can use the PCI-6251 with the NI BNC-2110 BNC termination block to interface the signal with the board.
The PCI-6251 works with multiple programming platforms, from the easy-to-use LabVIEW SignalExpress to the versatile LabVIEW or NI LabWindows™/CVI environments. NI hardware provides software support to further customize your application.
For a low-cost alternative, NI USB-6008 data acquisition (DAQ) devices offer basic measurement capabilities but are packed with many features and terminals for versatility and ease of use. You can measure signals directly from the compact, inexpensive USB-6008 DAQ device.
Using LabVIEW or LabVIEW SignalExpress with the USB-6008 delivers simple acquisition capability at a low cost, making this configuration the perfect option for basic applications.
A third method of current measurement involves the use of a Hall effect current transducer. By generating a magnetic field in the presence of current flow, you create a potential difference that directly corresponds to the magnitude of the current flow. You can measure and analyze this potential with a data acquisition instrument in a manner similar to the other measurement methods. Hall effect current transducers are not as invasive as the other methods of current measurement, but they are generally cost-prohibitive.
National Instruments Products for Power Measurement
National Instruments offers several options to measure the current across a shunt resistor easily, accurately, and reliably.
Good: The most basic yet cost-effective option is the USB-6008. It provides compact solution with simple connectivity to signal sources.
In addition, the USB-6008 works with LabVIEW and LabVIEW SignalExpress for quick and reliable data acquisition.
Figure 3. NI USB-6008
Better: The PCI-6251 high-speed multifunction M Series DAQ board provides the perfect balance between functionality and performance. It features 16-bit resolution, high-speed acquisition rates of 1 MS/s (multichannel), and the NI-MCal calibration technology for improved measurement accuracy.
The PCI-6251 is fully supported by LabVIEW, LabVIEW SignalExpress, LabWindows/CVI, and NI Measurement Studio for quick and reliable data acquisition.
Figure 4. NI PCI-6251
- NI PCI-6251 data acquisition board (Part #: 779070-01)
- NI SHC68-68-EPM shielded cable (Part #: 192061-01)
- NI BNC-2110 shielded connector block (Part #: 777643-01)
- SCXI Resistor Kit (Part #: 776582-01)
Best: If acquiring the most precise current measurement is the ultimate goal, the PCI-4070 high-precision DMM provides exceptional bit resolution, up to 6½ digits of measurement precision, and self-calibration capability to keep measurements accurate automatically.
With National Instruments proprietary FlexDMM technology, these DMMs also offer a fully isolated, high-voltage digitizer capable of acquiring waveforms at sampling rates up to 1.8 MS/s at all voltage and current values.
Figure 5. NI PCI-4070
- NI PCI-4070 high-precision DMM (Part #: 778275-01)
- P-1 Probe Set (Part #: 761000-01)
- SCXI Resistor Kit (Part #: 776582-01)
National Instruments Software
- LabVIEW 8.5 Full Development System (Part #: 776670-09)
- LabWindows/CVI (Part #: 776800-09)
- LabVIEW SignalExpress (Part #: 779037-09)
2. Shunt Current Monitor IC-Based Measurement
A preferred alternative to using only a shunt resistor in either high-side or low-side current measurement configurations is the additional use of a current shunt monitor integrated circuit (IC). These ICs are designed to convert current level into a voltage signal that is both safe for and measurable by an external data acquisition system. There are several advantages to using an IC for the measurement of the current flow through a circuit. In a high-side current measurement configuration, you can rate the IC to withstand the high common-mode voltage levels that may damage an acquisition device. In applications such as those involving power measurements of a microprocessor core, the current levels can be very small with corresponding small differential voltage levels (commonly in the low millivolt range). Cost-efficient data acquisition devices may not be able to accurately digitize these voltage levels because of measurement device specifications and because environmental noise can also contribute heavily to inaccurate measurements. Without affecting the behavior of the system under test, these current shunt monitor ICs provide some degree of amplification of the voltage signal into a level more accurately measured by the acquisition device.
One example of an application that uses this method for power consumption measurement of an embedded microcontroller is the Texas Instruments 5509 Series fixed-point DSP evaluation boards. The shunt device used on this evaluation board – the Texas Instruments INA139 high-side measurement current shunt monitor (functional diagram shown in Figure 6) – convert the small current levels drawn by the microprocessor core, input/output circuitry, and overall system to signals measurable by a low-cost NI USB DAQ instrument integrated into the evaluation board. While not necessary for an application involving current measurement for a microprocessor, this IC provides a high common-mode voltage range. The output from the IC is then fed into an operational amplifier for more signal gain before digitization. In addition to the data acquisition device, National Instruments provides robust data visualization and a logging application built with the LabVIEW application development environment. With this application and the integrated data acquisition instrument on the evaluation board, you can quickly determine the power efficiency of the TI processor under various operating conditions.
Figure 6. Texas Instruments INA139 Functional Diagram
3. EEMBC and Energy Benchmarking with NI Data Acquisition Hardware
Knowing how to measure the energy efficiency of a microprocessor is one step in a designer’s goal of finding a device that meets application needs. Because there are often many options available when selecting a processor, a designer must be aware of which manufacturers and models offer the best “bang for the buck” in terms of energy efficiency. The Embedded Microprocessor Benchmark Consortium (EEMBC), with support from National Instruments, currently offers a solution for standardizing energy efficiency measurements across different microprocessor manufacturers.
This EEMBC measurement suite, referred to as EnergyBench, provides data on the amount of energy a processor consumes while running EEMBC performance benchmarks. The energy consumption measurements that the EnergyBench suite makes while a processor is running these benchmarks are made possible by the use of NI data acquisition tools, specifically the EEMBC-recommended NI PCIe-6251 and PCI-6251 multifunction DAQ boards and LabVIEW software. The EnergyBench manual provides a detailed list of all recommended hardware for an essentially out-of-the-box solution for power efficiency measurement.
Figure 7. EEMBC EnergyBench Analysis Software User Interface
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