Budgeting Sensor FIFO Service Time After a Watermark Interrupt

A sensor FIFO watermark interrupt means that buffered data have reached a chosen level; it does not stop new records from arriving. The host must begin and complete enough reading before the remaining capacity is exhausted. Build that budget from the actual record stream and worst service delay.

Identify the FIFO counting unit

The ST LSM6DSOX data sheet defines watermark, full, and overrun status separately and describes FIFO records that include a tag with sensor data. Capacity expressed in records cannot be treated as the same number of bytes or complete multi-sensor frames.

List enabled accelerometer, gyroscope, timestamp, and other record types. Record their batching rates and any compression mode. The total record production rate may be greater than either individual sensor’s output rate.

Calculate the time remaining

For a fixed record size and rate, divide unused FIFO capacity at the watermark by the incoming record rate. This gives a first estimate of time until full if no records are removed.

In a hypothetical FIFO with 256 record spaces, a watermark at 192 leaves 64 spaces. At 800 records per second, that is 80 ms of unserviced capacity. This example is not a specification for the cited sensor; use the selected device’s actual capacity and counting rules.

Include transfer and scheduling overhead

Budget interrupt latency, time waiting for a shared bus, command bytes, status reads, data transfer, and any gaps between bursts. During a read, new records continue to arrive. The sustained drain rate must exceed the incoming rate for the buffer level to fall.

Evaluate the longest permitted host delay, not only average loop execution time. Flash operations, radio activity, or another high priority task can create an occasional delay much longer than the normal service interval.

Verify overflow detection and recovery

Log watermark time, FIFO level before and after reading, transferred records, and overrun status during a stress test. Preserve the documented status clearing behavior so a read does not erase evidence before it is recorded.

Deliberately exercise a bounded delayed service case on a test setup. Confirm that the software identifies lost or overwritten data and restores a valid record boundary. Do not present a discontinuous stream as evenly spaced complete data after overflow.

Save the record map, capacity units, timing budget, worst observed latency, and recovery behavior. Recalculate when enabling another sensor stream or changing its rate. A reliable FIFO configuration provides demonstrated time margin and visible loss handling. A working watermark interrupt or an empty buffer during a quiet bench run does not establish that all samples survive the host’s busiest operating state.