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Identify drain-current conditions when computing power of multicore SoCs

Posted: 17 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:power numbers? multicore? drain current?

For the dual-CPU SoC under consideration, there were various discussions held with systems, applications, and design teams. Some prototype experiments were conducted on silicon to accurately define the TRCs. The factors which were found to have direct influence on the Run-Idd numbers are:

LSM or DPM mode of operation: Selection of SoC mode of operation is mainly dependent on the customer use-case scenario. But for the worst case power consumption, because LSM mode has both CPUs executing instructions simultaneously all the time, the maximum core activity occurs in LSM mode and hence is TRC1 for the worst-case power measurement.

Frequency of operation: The CPUs in SoC under consideration work at a maximum frequency of 180MHz with the rest of the system operating at 90MHz. Experiments done on real silicon showed that the current numbers are directly proportional to the frequency of operation. The graph of figure 2 below depicts the dependency of core and I/O current on the CPU operating frequency.

Figure 2: Power vs. operating frequency.

So it was decided to take current measurements at the highest frequency of operation, i.e. 180MHz. This is TRC2.

Cache enabled operation: The enabling of the instruction and data cache allows faster CPU execution owing to lesser memory access time, which in turn should cause higher current consumption on the core voltage. This was established during our prototype experiments and hence is TRC3.

CPU execution time vs. peripheral execution time: For SoCs, the total execution time is divided between CPU execution time and peripheral execution time. Experiments done on real silicon showed that for a given condition, power consumption on core voltage is directly proportional to the CPU execution time, i.e. the higher the CPU execution time, the higher the core current consumption.

A quick look at the graph shown in figure 3 provides a clear picture of this dependency.

Figure 3: Power vs. CPU execution time.

To obtain the optimum numbers matching with typical customer use cases, we decided to keep the CPU-to-peripheral execution- time ratio as 80:20. This is TRC4.

Miscellaneous factors: There are many other factors which are SoC specific that can have considerable effect on the current consumption. For example, for the SoC under analysis, the CPUs implement complex signal-processing and FFT (fast Fourier transform) instructions. The execution of these instructions by a CPU has a direct impact on the core current consumption. For other SoCs, such factors having direct impact on the core current have to be identified and that becomes the TRC5.

Test case
Once the previous five run conditions were identified, a multimodule Run-Idd test case was developed. The SoC was configured in LSM mode with CPUs operating at 180MHz with I/D-cache enabled, executing complex FFT code with the signal-processing engine enabled, and checkerboard patterns on complete SRAM. The various modules enabled in the Run-Idd test case were ADC, eDMA, FEC, external bus interface (EBI), LinFLEX, watchdog timers, system timers, DDR controller, FlexCAN and DSPI. After performing the required system and module initializations, all of these modules and the CPU were configured to execute respective data transfers continuously in an infinite loop.

All the typical Run-Idd conditions discussed above should be clearly mentioned in the datasheet as shown with a line entry in the table here:

Table: Power parameter in a typical datasheet.

Power measurement conditions, setup
Publishing the datasheet with the final current numbers requires a thorough analysis by executing the defined Run-Idd scenario on SoC across process, voltage, and temperature. Generally, there are three samples selected each from slow, fast, and typical corner lots, two voltage conditions (nominal and maximum operating voltages, 3v3 and 3v6), and three temperature conditions (-40, 25, 125C).

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