Power measurement

Measure the energy an inference costs, not just its cycles. hpx captures current, voltage and energy around a GPIO-gated clean window: the firmware runs a dedicated, uninstrumented inference loop and drives a sync GPIO high for exactly that window, while an instrument integrates charge and energy and the host reads the result. Two instruments are supported, and the guides below cover setup, measurement windows and verification.

Prerequisites

A working PMU profile of the model (First profile), and one of:

• a Joulescope JS110, JS220 or JS320 in series with the board’s supply, with at least one GPIO wire from the board to the instrument; or
• a TI INA228 current monitor on the board’s own I2C bus, for whole-window energy without a bench instrument.

Which instrument

	Joulescope (power.driver: joulescope, mode: external)	INA228 (power.driver: ina228, mode: internal)
What it is	A bench instrument in series with the supply, watching a GPIO gate	A monitor chip on the target rail, read by the firmware over I2C
Streaming samples	yes	no; hardware accumulators
Whole-window energy	yes, HPX accumulates instrument statistics over the selected gate	yes, integrated on the chip
Current distribution (median, p95, p99)	yes	no
Host wiring	series supply plus one to three GPIO wires	none; target I2C
Powers and resets the target rail	yes (relay, power-cycle recovery)	no
Adds to the measured current	no	yes: the IOM that talks to the chip stays powered inside the measurement
Pick it when	You need external current statistics and GPIO-gated energy	You need on-target aggregate energy without a Joulescope

power.driver names the chip or instrument the firmware and host talk to. The registered drivers are joulescope and ina228 (src/helia_profiler/power/__init__.py); any other name raises PowerError with the list of available drivers. Use ina228 for an on-target measurement. Runs made with different instruments carry different measurement_scope values (gpio_gated_clean_window for the Joulescope, on_device_gated_inference for the INA228), and hpx compare omits power deltas between them.

How a power run works

A power runThe dedicated power binary runs a clean window; the instrument integrates it; the host verifies the window before it publishes a number.

• Power firmwareno transport
  • Warm-up
  • READY, GOlock-step
  • Clean windowgate high
• Instrument
  • Joulescopegate on INPUT0
  • INA228I2C, accumulators
• Host
  • Window checkstwo clocks agree
  • summary.jsonpower block

Text description of the diagram

• Power firmware: no transport
  • Warm-up
  • READY, GO: lock-step
  • Clean window: gate high
• Instrument
  • Joulescope: gate on INPUT0
  • INA228: I2C, accumulators
• Host
  • Window checks: two clocks agree
  • summary.json: power block

In order: hpx flashes a second, transport-free build of the firmware (power.firmware: dedicated, the default) that does model init, warm-up, the three-wire lock-step handshake and the gated clean window, then parks. With a Joulescope, the host watches the gate on INPUT0, sums the statistics packets that fall inside it, and divides the gated energy by the firmware-reported clean inference count. With an INA228, the firmware resets the chip’s accumulators, runs a fixed number of inferences, reads energy and charge back, and reports them through the same terminal record. Either way the host cross-checks the firmware’s own window clock against the instrument before it publishes per-inference numbers.

Steps

1. Joulescope setup and wiring→

Series supply wiring, the per-board GPIO pins for gate, state and GO, Vref on JS220 and JS320, io_voltage, and the lock-step handshake.

2. INA228 on the target→

Wiring a Power Monitor Click or an Adafruit breakout, choosing a shunt, what the on-device summary contains, and its failure modes.

3. Windows and the clean window→

The dedicated power firmware, window sizing and duration, reset strategies, and the advanced power floors.

4. Verifying a capture→

The power block of summary.json, the health signals, the window clock checks, bring-up diagnostics and troubleshooting.

Two topics other pages link to by their old anchors live on the Windows page: the dedicated power firmware and AOT input preparation.

The power block’s every key, type and default is on the generated configuration reference; the pages above explain what the keys do.

Expected output

summary.json gains a power block with measurement_scope, average and peak current, average power, the current percentiles (Joulescope), energy per inference, inferences per joule, and the health signals the verification page lists; the terminal prints a compact power table. The power examples provide configurations and explain how to read the output. Their validation sections identify which steps have captured evidence; they do not provide a validated power benchmark.

What next

Start with Joulescope setup and wiring if you have the instrument, or INA228 on the target if you do not.