Power Measurement Platform

Presentation

Evaluating software impact on power consumption is not easy. Using a simple multimeter on the power line is not accurate enough because there are big variation in input current. A higher sampling frequency is needed. Also you have to correlate the power data with the various phases of a software (separate initialization from the computation part for example).

Our idea is to build a low-cost power measurement tool around a cheap microcontroller, in this case from the pic18 family. We use its ADC to get current and voltage sample of the power line at a high enough rate. We also use its GPIOs as input, connected to GPIOs of the SoC being evaluated, to track and report various software state on the SoC.

We choose to use a pic18fj53. Its has a 12 bits ADC which can do up to 80 ksp/s, runs at 48 Mhz and has a USB interface. Our plan was to use the USB interface to transfer the data to the host. Unfortunately during development we found that the USB interface, when enabled, would dramatically decrease the ADC's accuracy. So we choose to use the serial port on the PIC side and added an external USB/serial bridge. The electronic gets power from the USB connection, the external power supply being used only for the SoC being monitored.

The requirements are:

• inputs between 0 and 2 A, 0 and 20 V,
• 4 GPIOs to connect with the SoC being evaluated.

We came up with the following schematic (PDF version):

Power measurement platform circuit diagram.

Hardware and software sources are available in the following SVN repository: https://www-soc.lip6.fr/svn/soc-conso/.

A 1.235 V precision diode is used as a voltage reference for the ADC. The input voltage is feed to ADC input 0 (AN0) via a voltage divider. To cover the input range with enough accuracy 3 different values can be selected with a jumper.

3 Shut resistors are used for current measurements, selectable with a jumper. An amplifier will multiply the shut's voltage before feeding it to AN1.

RB0-RB3 are used to get 4 GPIO status from the SoC.

Each sample reported to the host is composed of 3 values: GPIO state (integer between 0 and 15), voltage (AN0 value between 0 and 4095) and current (AN1 value between 0 and 95), in the format: gvvvcccX, with g, v and c being hexadecimal digits and X being the letter X used as separator. With 8 bytes per sample, at 921600 bauds we could report in theory up to 11520 samples/s. To be safe we choose to run at 5000 samples/s (which requires 10000 A/D conversions/s).

Calibration and Performance

For the calibration process, I used a good linear power supply (both current and voltage adjustable) and a 4000 points digital multimeter. A switching power supply was not good enough, ripple noise impacted values reported by our circuit.

For voltage input, we selectable 3 divider values: 4.92, 11 and 19.33, which gives ranges of 6.07 V, 12.35 V and 23.88 V respectively.

For current input, we have 3 selectable shuts: 0.05, 0.1 and 0.2 ohms. The shunt's voltage is then amplified by 10 by the AD623 amplifier. This gives ranges of 2.45 A, 1.23 A and 0.61 A respectively.

For each tension or current range, we take 8 samples between 0 and a value close to the max of the range. For each sample we get a few thousand values from pic's output, while we note the real value using the digital multimeter.

From this process, we found that that the AD623 output can't go down to 0, it stays positive by anout 5.5mV. This means we can't measure currents smaller than 2.75 mA on the 0.61 A scale, and 11 mA on the 2.45 A scale. We also find that, assuming an ideal power supply without noise or ripple, we have a +/-7 unit error on the ADC's output for current values. This is less than 0.18% of the full range (or +/- 4.4 mA on the 2.45 A range input). Below are plots for measures on the 0.61 A scale.

ADC values for 0 mAADC values for 536.7 mA

From the 536.7 mA measure we have a 8% error for the ADC's output (we expect 3559, we get 3851). We can use this to correct our conversion ratio: \(adc(i) = 3851 / 536.7 \times i\) (with \(i\) in mA). With this conversion function, and excluding the value at 0 mA, the error with the measured ADC output is at most 4, which is within the noise.

Current input to ADC output.

We repeated the process for the tension inputs. Here we have a +/-5 unit error on the ADC's output for voltage inputs. This is less than 0.13% of the full range (or +/- 29 mV on the 23.88 V range). Below are plots for measures on the 6 V scale.

ADC output for 0.171 V input ADC output for 5.996 V input

For 5.996 V, we expect 4045 at the ADC output and we did read 4041 0r 4042. This is less than 0.1%. As for current we use the 5.996 V result to fix our conversion ratio: \(adv(v) = V \times 4041.5 / 5.996\). With this function, the error with the measured ADC output is at most 5.

Tension input to ADC output.

Version 2

The version 2 of the hardware got several improvements:

• As the USB part of the pic18 is not used, we switched to a pic18f27j13
• There are now 8 GPIOs available.
• A 4th current range (up to 5 A) has been added.
• The USB/serial part is electrically decoupled from the target board, to avoid possible ground loop affecting current values. The USB/Serial bridge is powered from USB, while the pic18 is powered from the load power supply.
• The pic18 can power on or off the load.
• The pic18 controls two solid state relays, which can drive e.g. power or reset switches for the target board.
• The pic18 has one optocoupled input, which can monitor e.g. a power led from the target board.
• A second serial port is available from the pic18, which can be used to connect to a TTL-level serial console on the target board, avoiding an extra serial/USB converter.

The software also got major reworks. To use the new features, a bidirectional connection is now needed. We also used this opportunity to store calibration value in the pic18's flash, so we don't need to match a board with its calibration values on the host's side. The USB/serial port of the board runs at 921600 bauds. The board accepts the following commands from the host:

• B<n> connect to the target's serial port at speed \(n\). If \(n\) is omitted, use the previously set speed. exit with #.
• C get calibration data.
• C<string> set calibration data string. It is just a string stored in flash and echo'ed back on request; it's up to the host software to interpret it. In order to keep the firmware simple, the string has to be exactly 83 char longs, and can hold only digits, . or spaces.
• E causes the firmware to exit, resetting the pic18.
• Mn with \(n\) being 1 or 0: start or stop measures.
• Pn with \(n\) being 1 or 0: power on or off the load.
• Rrn with \(r\) being 1 or 2 and \(n\) being 1 or 0: turn on/off relay \(r\).
• S get various input states.

Each command is replied with OK, unless and error occurred.

Host software getting values from the board now has to start measures with M1 first; it should also send M0 before exiting.

Configuration of the Measurement Platform

This section explains how to configure the measurement platform (version 2) for a new SBC.

Measurement Platform v2 set on 5 V + 4 A input.

The Platform possesses two jumper types :

• the red one : controls supported voltage with 3 possible positions,
• the black one(s) : controls supported intensity with 4 possible positions.

Voltage (red) :

• position 1 : from 0 to 6.07 V,
• position 2 : from 1 to 12.35 V,
• position 3 : from 2 to 23.88 V.

Note

It's best to use the interval closest to the supply voltage to improve measurement accuracy. For example, for a voltage of 5 V, all positions could be used, but position 1 will result in a more accurate measurement of energy consumption.

Intensity (black) :

• position 1 : max 0.61 A,
• position 2 : max 1.23 A,
• position 3 : max 2.45 A,
• position 4 : max 4.90 A.

Warning

In positions 1, 2 and 3, a single black jumper is used, while in position 4 both black jumpers need to be used.

Configuration on Brubeck :

• /sog/boards/boardX/tension: must contain the number corresponding to the number of the voltage jumper (red) on the measurement platform
• /sog/boards/boardX/current: must contain the number corresponding to the intensity jumper (black) on the measurement platform

Note

X corresponds to the number of the measurement platform. These numbers are written on labels in the Monolithe.

Setting up GPIOs

The GPIOs control the first value of each measured sample, thus allowing to identify specific parts of the running benchmark. Three steps are mandatory to set up a GPIO on the operating system.

1. Export the desired pin in the sysfs :

   echo DRIVER_GPIO | sudo tee /sys/class/gpio/export
   

   where DRIVER_GPIO is the Pin ID given by the driver (column Driver GPIO ID of the next table). For instance, on Jetson Orin Nano, DRIVER_GPIO is 453 for the pin 29.

2. Set the pin direction

   echo out | sudo tee /sys/class/gpio/SYSFS_GPIO/direction
   

   where SYSFS_GPIO of the previous example is PQ.05.

3. Set write permissions and add symbolic link

   sudo chmod o+rw /sys/class/gpio/SYSFS_GPIO/value
   sudo ln -s /sys/class/gpio/SYSFS_GPIO/value /sog/gpio/pinX
   

   where X is the number of the pin in table below (usually 1, given /sog/gpio/pin1).

Connected pins

Board Name		Pin 1
	GPIO Pin	Driver GPIO ID	GPIO SYSFS
Odroid-XU4	26 (GPIO24)	24	gpio24
Jetson TX2
Jetson AGX Xavier	22	417	gpio417
Jetson Nano	19 (SPI_1_MOSI)	16	gpio16
Raspberry Pi 4 Model B	40	21	gpio21
Jetson Xavier NX
Jetson Orin NX	29 (GPIO01)	453	PQ.05
Jetson AGX Orin	13 (GPIO32)	456	PR.00
Jetson Orin Nano	29 (GPIO01)	453	PQ.05
Orange Pi 5 Plus	10 (GPIO1_A0)	32	gpio32

Install powerstat on EM780 & Ubuntu 24.04 LTS

sudo apt install powerstat

To use powerstat in user mode:

sudo apt install sysfsutils
sudo chmod -R a+r /sys/class/powercap/intel-rapl
echo "mode class/powercap/intel-rapl:0/energy_uj = 0444" | sudo tee --append /etc/sysfs.conf

Run powerstat in user mode:

powerstat -cDHRf 2