Measurement and averaging

What one read returns, how often to read, and how the sensor's averaging and IRQn line change that.

The measurement loop

After a start command the sensor measures continuously, about every 0.5 ms (2 kHz). Each read of the result buffer returns the latest value of three signals: flow, temperature and the status word. The flow is averaged on the sensor in one of two modes; the temperature updates at about 90 Hz.

stateDiagram-v2
    [*] --> Idle: power-up or soft reset
    Idle --> Measuring: start command (12 ms to first result)
    Measuring --> Measuring: read, update concentration
    Measuring --> Idle: Stop() (up to 0.5 ms)
    Idle --> Sleep: EnterSleep()
    Sleep --> Idle: ExitSleep() (typically 16 ms)

Only idle mode accepts configuration commands: averaging, scaling, product identifier and sleep. While measuring, the sensor takes stop, soft reset and the concentration update, and nothing else.

Average-until-read (default)

ConfigureAveraging(0), and the mode after power-up and after a reset. The sensor averages every sample since the previous read, so a read at any rate returns the mean over the whole interval: a fixed-rate poll is anti-aliased without a filter on the host.

For the first 64 ms after a read the average is the arithmetic mean. If you wait longer, the sensor switches to exponential smoothing with α = 0.02 (status bit 11 set), which weights roughly the last 64 ms. Read at least every 64 ms to keep the plain mean.

Fixed-N averaging

ConfigureAveraging(N) with N from 1 to 128: each result is the mean of N samples, so a new result is ready every N × 0.5 ms. Reading earlier gets a NACK (DriverError::NoData). N = 1 turns averaging off. Status bit 10 is set in this mode.

N	New result every	Rate
1	0.5 ms	2 kHz
4	2 ms	500 Hz
20	10 ms	100 Hz
128	64 ms	about 16 Hz

Configure averaging in idle mode, before StartContinuous(); the setting holds until a reset or the next ConfigureAveraging(). The driver accepts 0 to 128 and returns InvalidParameter above.

The IRQn line

IRQn goes low when a new result is ready (at 2 kHz, or 2 kHz / N with fixed-N averaging) and returns high when the result is read. It is also low after power-up or a reset, until a start command. So a low IRQn after a start means "result ready", unless the sensor was reset, in which case the read is NACKed.

Use it to read only when there is data: wait for the falling edge in a GPIO interrupt, read the 9-byte frame there (or start a DMA transfer) and decode it with Driver::DecodeMeasurement(), which needs no bus access. The API guide shows it. Leave IRQn unconnected if you poll.

What "NoData" means

A read header with no new result is NACKed by design. The driver reports it as DriverError::NoData: skip the tick and read again later. It is a fault only when it never stops, which means the sensor is idle (no start, or it was reset) or the address is wrong.

Start-up

Time after the start command	State
0 to 12 ms	No result; reads are NACKed. StartContinuous() waits these 12 ms
12 ms to 30 ms	Results arrive but can be a few percent off while the heater warms up
From 30 ms	Results within the datasheet accuracy

Changing the gas with a new start command costs the same 12 ms, and 70 ms until the reading settles. To change only the O₂ fraction of a mixture, use UpdateConcentration(): it does not interrupt the measurement.

Next: Gases and part families.