LSM6DSOX/ISM330DHCX: real sample rate lower than expected in FIFO mode

[GHope.2]

Hi,

I am using the LSM6DSOX and ISM330DHCX in FIFO mode at 52 Hz or 208 Hz. For 52 Hz the real frequency turns out to be about 48.57 Hz, and proportionally so for 208 Hz. I am confirming this through the number of samples collected between GPS fixes and when comparing a signal against another type of sensor. The observed frequency is stable at this rate.

Could this be real or is there something wrong in my application? The FIFO is checked using an RTC and is never allowed to run full (the overflow, full, overflow_latched) bits are checked. I drain (pop) the number of samples available at the RTC interrupt, but do not reset it. Does operating the FIFO or draining it halt the FIFO or reduce the number of samples?

I observe the same both with the LSM6DSOX and the ISM330DHCX.

IMU configuration: github.com/gauteh/sfy/blob/main/sfy-buoy/src/waves/mod.rs#L231

FIFO drain function: github.com/gauteh/sfy/blob/main/sfy-buoy/src/waves/mod.rs#L395

Best regards, Gaute

[Federica Bossi · Accepted solution]

Hi @GHope.2​ ,

Welcome to ST Community!

You are not making any errors in your application, consider that the spread can span the +-10% range so having 48.57 Hz instead of 52Hz is acceptable.

If my reply answered your question, please click on Select as Best at the bottom of this post. This will help other users with the same issue to find the answer faster :)

[GHope.2]

Thank you, that is a relief. Is the spread stable, i.e. will the frequency for an individual chip always remain the same? I've observed that a batch of instruments have apparently identical sample rate (48.57 Hz), but they may be from the same production batch. Is that still realistic?