[FXOS8700] Transient Detection troubles

Ruig25 — Tue, 03 Feb 2026 21:31:43 GMT

Hello!

I know, that the sensor is a little bit old, but nevertheless.

I have one application case and I still cannot solve a problem with transient detection. Of course I have read all App. Notes.

Use case description:

Sensor board attached to the wrist so that x-axis is along the arm (if we stretch index finger - the x-axis and the finger are +/- parallel).
The arm is on the rest (Xg =~ -250mg). It is initial position.
Now the stretched arm will be rosen slightly above the head (Xg =~ 300-350mg).
X and Z axes are ignored (out of interest).
I want to have an interrupt after hand was rosen as described above.

Here is described motion (ODR=100Hz)

My first idea was to use vector-magnitude detection. But I did't tried it because I don't have constant Gref (initial position may have some variiations...).

The second idea - motion or transient detection.

Code (abstract):

software rest + 1ms wait
Standby CTRL_REG1=0x0
Fullscale 2g XYZ_DATA_CFG=0x0
Clr debounce counter and set ths to 5 (5x63mg=315mg) TRANSIENT_THS=0b10000101
TRANSIENT_COUNT=6
X only + evt latching TRANSIENT_CFG=0b00010010
Enable transient interrupt CTRL_REG4=0b0010000
Route transient interrupt to int1 CTRL_REG5=0b00100000
Active mode, ODR 12,5Hz, Low Noise CTRL_REG1=0b00101101

Interrupt event triggered non-stop. I tried many different values for 4) and 5).

uC input IO configured for neg. edge with enabled pull-up.

I tried to move 6) after 3) like here. No interrupt triggered at all.

I tested circuit enabling FIFO watermark interrupt. Works without problems.

Any ideas?

Or motion detection is more suitable in my case?

Many thanks in advance

Re: [FXOS8700] Transient Detection troubles

Ruig25 — Mon, 29 May 2023 08:30:04 GMT

After 10h of investigation:

It seems that threshold set in TRANSIENT_THS is ignored. Therefore interrupt fired non-stop (fallback threshold 0mg?).

But after enabling a_ffmt_trans_ths_en=1 in A_FFMT_THS_Y_MSB the system started to work correctly.

Axis of interest is X.
And I didn't expect to use invidual thresholds per axis. But it works now.

Re: [FXOS8700] Transient Detection troubles

JozefKozon — Mon, 29 May 2023 10:10:26 GMT

Hello Igor,

for your case, I think the best would be the "Acceleration vector-magnitude detection". Please refer to the section 12.10 in the FXOS8700CQ datasheet and to the AN4692.

With Best Regards,

Jozef

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[FXOS8700] Transient Detection troubles

Re: [FXOS8700] Transient Detection troubles

Re: [FXOS8700] Transient Detection troubles