How to automate SoC current measurements
Today’s SoCs (system on chip) have many modes of operation. Subsystems are shut down when not in use. When the entire SoC is in a "sleep" mode, many subsystems may be shut down or run on minimal power. That, combined with the fact that an SoC may have several power supplies, means testing for current consumption requires many steps and measurements. Measurements must be repeated for each state of operation, which can be tedious and time consuming if any part is done manually. Automated current measurements under computer control using a multiplexer can save test time and produce repeatable measurements.
While measuring the SoC Current consumption, we use an ammeter in series with the power supply to measure its output current. This sounds simple, but you must take proper care when setting the ammeter’s current range. The current range used while measuring can’t be lower than actual value to get a measurement. At the same time, the range must not be too high because the ammeter won’t provide sufficient resolution. Always choose the most immediate higher range above the actual measurement value. Furthermore, the point in time and length of ammeter range switching is also important for proper device functionality. Automating the ammeter’s operation can assure that the meter is properly set for each set of measurements.
SoC operating modes
To achieve additional power savings, an SoC works in different modes of operation. It is imperative to switch off most of the peripherals or modules if they’re not required to operate in the sleep mode. There are many more Power saving modes in the latest SoC devices depending on the requirement of different functions in different modes. Current consumed by the device in all these modes is mentioned in the datasheet of the device. Examples of SoC modes: Run Mode, Stop mode, Halt mode, Sleep mode, Standby mode, and others.
Current consumed by SoC typically consists of Core Current and IO Current. Core current is the current consumed by all low-voltage transistors which constitute core logic, SoG (Sea of gates) & circuitry of SoC. Normally, this constitutes the major portion of the total current consumed by the device. IO supply current consists of current consumed by the IO pads and its structures in SoC. Usually IO current is due to High Voltage transistors since the external world works on 3V/5V levels. A typical current distribution ratio of Core and IO currents during Run mode is 20:1.
We’ve also found a need to analyze the current distribution, particularly in low power modes, which helps in determining which SoC functional block or IOs consume more current. Then, we can look for ways to reduce it. The following graphs show typical Core Current (Figure 1) and IO Current (Figure 2) variation with the change in voltage and temperature for a typical SoC. Note: All the graphs shown in the present document are for illustration purpose only and do not show any actual SoC numbers.
Figure 1. This plot shows an example of an Soc’s core current with respect to Temperature and Voltage.
Figure 2. This plot shows an example of IO current with respect to temperature and voltage.
Core Current (as shown in Fig. 2) shows a consistent rise with increase in temperature, but remains almost constant with the voltage. The rise in core current at higher temperatures is due to an increase in leakage current of transistors depending on the manufacturing process (28 nm/45 nm) or technology used and the wafer corner lot. IO supply current, however, shows a different behavior (Figure 3). It rises with the increase in voltage and temperature but percentage change in the IO current is much less. Current consumed by the device increases linearly with rise in system frequency.
Figure 3. Variation of Total current with System frequency for three samples.
Manual and automated measurements
SoCs typically use multiple power supplies. A typical SoC used in automotive applications may have ten or more voltages. SoC low-power mode adds further challenge to automated current measurement setup. As a result, these current measurements are done manually or only partially automated.
With manual automated measurements, a computer may actually conduct acquire the measurement data, but a person is required to initiate them and configure the ammeter for each test and connect it. That results in long execution time due to Soak time required at every temperature. The LabVIEW code loading time that we use for every mode and DUT programming time for every mode can get rather long when manually loading and initiating tests. A person must set the appropriate ammeter ranges manually for different SoC power modes. In this approach we face another problem that we need to have as many ammeters as many number of SoC power supplies as is shown in Figure 4. Should we not have enough ammeters, then we need to manually insert an ammeter one-by-one on all power supplies in order to complete measurements across supplies.
Figure 4. In a manual current measurement, an ammeter is inserted in line between a power supply and SoC under test.
In our automated method, we implement a multiplexer, which lets us automate test setup across many SoC power supplies and covers all test cases with one ammeter. The multiplexing system lets us switch a single ammeter from one supply to another until measurements are completed. It solves the problem of manually connecting one ammeter on different supply channels. Figure 5 shows the complete algorithm, which includes complete sequence of automation.
Figure 5. This flowchart shows the setup decision process for an automated current measurement station for SoCs.
During a test, the SoC is programmed once, such that it has code for all SoC Power modes with appropriate mode-selection logic. SoCs always power up in normal-run mode and wait for test case selection command. As per each test case, the selection command SoC Power mode is selected for the current measurement. After measuring the current in the selected power mode, SoC will be powered down and the process repeats for the next power mode.
To complete measurements across multiple modes, we select the required SoC logic case that corresponds to the power mode in which current is to be measured. This case selection can be done using either a serial communication interface (RS-232) or through GPIO ports. Communication between the computer, ammeter, and multiplexer is through GPIB.
Through communications, we can automatically select the voltages, temperatures and the SoC modes. With the multiplexing system, we can select the supply channel at which to measure current. In this way automating the setup for all the test cases required for measuring current.
Figure 6 shows a top-level block diagram measuring current across multiple SoC supplies using one ammeter and a multiplexing system. The path corresponding to the power supply (the supply for which we want to measure current) is opened and the ammeter is inserted in series for that power supply. Paths for the rest of the power supply are closed. Before taking the measurement, the software will select the appropriate ammeter range.
Figure 6. Hardware Setup for automated measurement shows the power supplies and multiplexer switches.
Comparison between test methods
The major advantages of the automated method of Current measurement are:
- No need to program DUT every time for a new power mode
- No need to manually insert ammeter or select range as per current expected
- Only one ammeter with one multiplexing system to measure current across all SoC supplies
- Temperature Soak time saving in Automated method
We also performed a time comparison between two methods. We found that the manual method needed about five days to complete. Using the automated method, the test need just one day. The comparison is based on an SoC that has five power supplies and nine power modes.
Automating the current measurements saves time and improves accuracy when making measurements done across multiple modes of SoC (including low-power modes) and across multiple supplies. Moreover, it makes acquiring data easy and independent of human intervention.
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