Checking PMBus Telemetry Format Before Converting Raw Words

A PMBus read can complete successfully while the displayed voltage or current is wrong because the host decodes the returned word using an inappropriate numeric format. Command names identify quantities, but the device documentation defines how those quantities are encoded. Validate the conversion alongside the electrical measurement.

Build a command-specific format map

The TI INA233 data sheet specifies DIRECT format for its telemetry and provides conversion coefficients. Current and power scaling depend on the selected current LSB and calibration, while some voltage scaling is fixed.

For every used command, record byte count, signedness, byte order, format, units, and any coefficient source. Do not apply a LINEAR11 decoder merely because another PMBus device uses that format for a similarly named command.

Check the arithmetic independently

For the DIRECT convention described by the device, the engineering value is obtained by scaling the received integer by the decimal exponent, subtracting the offset coefficient, and dividing by the slope coefficient. Preserve signed values and use a numeric range wide enough for intermediate calculations.

As a hypothetical example with slope 2, offset 0, and exponent minus 1, a raw integer of 100 converts to 500 in the defined engineering unit. Reversing the exponent sign would produce a very different answer despite an identical bus transaction.

Match calibration to installed hardware

Confirm that the programmed calibration matches the actual shunt resistor and the intended current range. Record when calibration is loaded after reset and whether the device retains it through power state changes.

A software scaling factor cannot compensate for an uninitialized hardware calibration register unless the complete measurement relationship is understood. Check voltage telemetry separately from current telemetry so a shared display bug is not confused with a shunt configuration error.

Compare with controlled electrical values

Apply several known operating points and compare raw words, decoded values, and independent instrument readings. Include zero and both current directions if the device and application support them. A single positive test value may not reveal incorrect sign extension.

Verify warning threshold encoding separately where those registers have different resolution or comparison rules. Reading a correct current value does not prove that an overcurrent warning has been programmed to the intended physical level.

Save the command map, coefficients, calibration value, raw captures, and reference measurements. Repeat after a reset to expose missing initialization. A reliable telemetry path provides a traceable conversion from physical quantity to raw word and back. A plausible number on a dashboard, or an acknowledged PMBus command, is insufficient evidence that the board is reporting the correct voltage, current, or power.