Trace: • power_measurement_on_lager
Power management optimization is becoming a key acceptance criteria for modern high-performance embedded application processor like Renesas RCar-H2.
In order to assess and validate software power management optimizations, a platform shall provide power measurement capabilities.
The purpose of this page is to describe how to enable such power measurements on RCar-H2 Lager Board, by analyzing the board schematics and layout, highlighting where probe points could be added.
This page will start first with a short description of what power measurement means, then analyze the Lager board capabilities with regards to power measurements (with no or minimal board modifications), and conclude with recommendations for next generations.
Note this study was intended to enable the monitoring of the RCar-H2 processor power consumption, but not other external peripherals present on the lager board.
Any hardware modifications described below are done AT YOUR OWN RISK. Baylibre SAS shall not be held responsible and/or accountable of any board damage or alterations that might occur.
To do power measurements, one needs to be able to measure both the voltage and current:
Additional notes about power measurements:
To measure current consumption, a shunt resistor shall be placed in series with the power line. Following Ohm's law ( U = R * I, i.e. I = Ushunt / Rshunt), by measuring the voltage drop at the resistor ends and knowing the resistor value, the current can be calculated.
Additional important notes about current consumption:
Contrary to current measurement, measuring the voltage usually does not require any extra onboard component, but only 2 test points for the voltage measurement equipment:
RCar-H2 processor Lager board schematics and layout was fully reviewed in order to:
Please follow the 2 links below to download 2 representations of the RCar-H2 processor Lager board power distribution:
For each power connection, we reviewed the schematics and layout. We used 3 different colors to rank the board modification level of difficulty:
NB: regarding voltage measurement, the technique is identical for each power rail, so it will not be explicitly described below. One end of the voltage probe shall be connected to the Lager Board ground, the other to the shunt resistor end, the one that is the closest to H2 processor.
H2 Cortex-A15 CPU0/CPU1/CPU2/CPU3 & Peripherals (SCU, L2$ controller) supports DVFS (Dynamic Voltage & Frequency Scaling) and are powered by VDD_DVFS voltage rail. Therefore this is a critical rail for CPU power optimization. The Lager board is already equipped with a shunt resistor (R584, 10mohm, 3W). hence the only remaining thing to do is to connect the measurement equipment to the R584 ends. This is pretty much the best possible situation, as:
Only missing item would be additional test points to avoid having to solder wires on shunt resistor.
This rails powers H2 Cortex-A7 CPU4/CPU5/CPU6/CPU6 + other peripherals (RCar-H2 documentation does not explicitly list all the internal peripherals connected to this rail), and SATA[0:1]. The Lager board is already equipped with a shunt resistor (R601, 5mohm, 0.5W) on this rail. Hence the only remaining thing to do is to connect the measurement equipment to the R601 ends. However even if the Lager board is equipped with the necessary shunt resistor, still the measurement on this rail will be difficult to analyze, as it powers many different peripherals at the same time. In practice, it will not be possible to distinguish (isolate) consumption from individual peripherals; but it will still be possible to track that peripherals are powered on/off on purpose at runtime.
This rail is used to power several Lager board peripherals, including RCar H2 processor (VD331, VCCQ, VCCQ_SD1, VCCQ_SD3).
Power rails dedicated to RCar H2 processor (VCCQ_SD1, VCCQ_SD3, …) are equipped with 0ohm resistor (R71, R74, R76) that can be easily replaced by shunt resistor. Therefore, Lager board does not limit the power assessment of RCar H2 processor.
However, when considering external peripherals, power measurement is considered of high complexity:
Note R602 (5mohm, 0.5W) shunt resistor can be used to measure global power consumption on this rail, with the limitation of not being able to isolate individual contributions.
This rails powers many H2 peripherals (DDR3 controller (DBSC), SATA[0:1], CPG, Display Unit (DU, including DU_LVDS[0-1]), MLB+, MLB, USB2.0 Channel [0:2]). The Lager board is already equipped with a shunt resistor (R603, 5mohm, 0.5W) on this rail. Hence the only remaining thing to do is to connect the measurement equipment to the R603 ends. In addition to this “global” R603 shunt resistor measuring the total current consumed, it also features a 1ohm resistor (e.g. R63) on almost all “individual” rails. Hence it is possible to both retrieve the total power consumption on this rail and isolate sub-rails consumption, with no board modification.
Note however that some rails like DDR_MSTB_VREF, VCCQ18, VD181, VDDQ_M1A, oscillators and glue components have direct wiring, making it impossible to isolate from others.
Note also that rails VDDA_SATA[0-1] & AVDD does not feature a shunt resistor but an inductance. Hence an additional shunt resistor will be required, so board modification complexity is raised to medium for these rails.
Having such fine-grained measurement is always appreciated, but may then become tedious when a single peripheral gets powered by several different lines, like for example the DDR3 controller (14 rails). In such situation, our recommendation would be to wherever possible add one extra shunt resistor per peripheral, to be able to measure peripheral total consumption in one shot.
On this specific D1.8V rail, one last recommendation would be to keep H2 processor and other external Lager board peripherals power rails isolated, i.e. use D1.8V_PERI to supply oscillators and Lager glue components/pull-ups.
This voltage rail is used to power some H2 board peripherals like HDMI, Video decoder, SDCard, oscillator. Power measurement of this rail is possible, but requires a little bit more work, as no shunt resistor is available:
But as none of these rails powers H2 processor, this higher level of complexity is not a limiting factor in terms of H2 power optimization.
This rail is not measurable. As a consequence, it will not be possible to do IO leakage optimization (impossible to track miss-configured IO, shortcuts, double pull-ups, pull up/down conflicts, …). We strongly recommend to add power measurement capability on this rail, as leakage optimization is key in overall platform power optimization.
These rails are used to power SDCard0 & SDCard2 peripherals of H2 processor. Mounted 0ohm resistors like R75/R101/R77/R89 could be replaced by shunt resistors, so it is considered easy to do power measurement on these rails.
This rail is not measurable, but is not used to power RCar H2 processor. Therefore it shouldn't impact H2 processor power optimization work.
This rail is used to power several Lager board peripherals, including RCar H2 processor (VDD_DVFS, VDDQ_M0BKUP).
Power measurement of Lager board peripherals on this rail is of medium or high complexity. Some are directly wired to the power source (e.g. LVDS_MON_POWER, SDIF_POWER, …) or require an extra shunt resistor to be inserted using inductance pads (e.g. L2, L6, …).
On the other hand, power rails dedicated to RCar H2 processor (VDD_DVFS, VDDQ_M0BKUP) are equipped with shunt resistors (R584) or 0ohm resistor (R93), making it easy to do power measurements.
This voltage rail is dedicated to powering RCar H2 processor external memories (DDR3). It features a shunt resistor (R585, 10mohm, 3W), hence enabling power measurement.
However, 4 rails out of 5 (DDR_VREF, VDDQ_M0, VDDQ_M1, and DDR memories) have direct connection with no 0ohm resistor nor test points. Hence only the total consumption will be measured, individual contributions can not be. This could be problematic with regards to DDR power optimizations, as there will be no way to make sure DDR low-power modes will be entered on purpose.
The Lager board is actually pretty well designed and enabled for power measurements: critical power rails do already feature the necessary shunt resistor. Hence we believe that this board can be used for RCar H2 processor power optimization work, with no sophisticated hardware rework.
Nevertheless, some areas may still be improved for next generations:
Finally, 2 additional global recommendations: