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Read power consumption measurements from watt meter Nets through the Lager CLI. Supports Yocto-Watt and Joulescope JS220 hardware.

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Syntax

lager watt [OPTIONS] NET_NAME

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Options

OptionDescription
--box BOXLagerbox name or IP address
--helpShow help message and exit

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Arguments

ArgumentDescription
NET_NAMEName of the watt meter net to read (optional if default is set)

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Usage

lager watt NET_NAME [--box BOX]
If NET_NAME is omitted and no default is set, lists all available watt meter nets on the box.

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Output

Returns power in watts with 3 decimal places: 
Power 'POWER_METER': 5.230 W
The reading timeout is 30 seconds. If no reading is received within that time, the command exits with an error suggesting the device may be disconnected or experiencing USB issues.

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Supported Hardware

ManufacturerModelIdentificationFeatures
YoctopuceYocto-WattUSB VID:PID 24e0:002aReal-time power measurement
JoulescopeJS220USB VID:PID 16d0:10baHigh-precision power, voltage, and current measurement

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Hardware Feature Comparison

FeatureYocto-WattJoulescope JS220
Power reading (read)YesYes
Voltage reading (read_voltage)NoYes (Python API)
Current reading (read_current)NoYes (Python API)
Combined reading (read_all)NoYes (Python API)
Measurement methodInstantaneous0.1s averaged
Device selectionChannel-basedSerial number-based
The CLI lager watt command returns power (watts) for both devices. The Joulescope JS220’s additional measurement capabilities (individual voltage and current readings) are available through the Python API.

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Instrument Name Matching

The backend driver is selected based on the instrument name in the net configuration:
PatternDriver
Contains joulescope or js220 (case-insensitive)Joulescope JS220
All other watt meter instrumentsYocto-Watt

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Default Net

To avoid specifying the net name each time: 
lager defaults add --watt-net POWER_METER
Then: 
lager watt

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Examples

# Read power from watt meter
lager watt POWER_METER --box my-box

# Read using default net
lager watt

# List available watt meter nets
lager watt --box my-box

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Scripting Examples

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Power Threshold Check

#!/bin/bash
# Verify power consumption is within limits
RESULT=$(lager watt POWER --box my-box)
echo "$RESULT"

# Extract numeric value for comparison
POWER=$(echo "$RESULT" | grep -oP '[\d.]+(?= W)')
if (( $(echo "$POWER > 10" | bc -l) )); then
    echo "FAIL: Power consumption too high: ${POWER}W"
    exit 1
fi
echo "PASS: Power within limits"

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Power Profiling

#!/bin/bash
# Sample power over time
BOX="my-box"
NET="POWER"

echo "timestamp,power_w"
for i in $(seq 1 10); do
    RESULT=$(lager watt $NET --box $BOX)
    POWER=$(echo "$RESULT" | grep -oP '[\d.]+(?= W)')
    echo "$(date +%s),$POWER"
    sleep 1
done

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Troubleshooting

ErrorCauseFix
Timeout (30s)Device disconnected or USB issueCheck USB connection; replug device
Connection refusedBox service not runningCheck box: lager hello --box <box>
Device not foundWatt meter not detectedVerify device is connected: lager instruments --box <box>

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Notes

  • Power is returned in watts (W) with 3 decimal places
  • Joulescope JS220 averages measurements over 0.1 seconds for higher precision
  • Net names refer to names assigned when setting up your testbed
  • Use lager nets to see available watt meter nets
  • Use lager instruments --box <box> to verify the device is detected