The frequency synchronization is possible if both IMUs sample at the same exact rate, then the ODR will not drift. This can be achieved by using a common external clock on CLKIN of ICM-4268x devices. For the phase synchronization both IMUs must sample at the same instant in time. This requires same clock, same reset/start event and deterministic enabling of sensors.
The ICM-4268x supports CLKIN frequency range of 31 kHz to 50 kHz. Typical choice of clock is 32.768 kHz crystal or clock generator. The clock needs to have a low jitter, preferably <= 50ppm, CMOS-level square wave and the same source need to be routed to both IMUs. For both ICM-4268x devices tie CLKIN (Pin 9) together. Keep trace lengths matched (important for phase). Follow the rise/fall time per RTC timing spec in the datasheet. Do not date or buffer one clock differently than the other unless you fully control propagation delay.
Step 1 - Power-up both devices together, apply VDD and VDDIO simultaneously. Keep both IMUs in Sleep mode initially. Ensure external clock is already running before configuration. The reason is that the internal clock selection logic locks early during start-up.
Step 2 - Select external clock as reference and configure both devices identically. Set clock selection to external CLKIN. This forces ADC sampling, Decimation, ODR timing to be derived from the same reference.
Step 3 - To minimize the phase error reset the signal paths by writing into SIGNAL_PATH_RESET register which clears ADC pipelines, digital filters and decimation states. Reset both IMUs, wait for the same delay a few clock cycles then proceed immediately to the next step. This step ensures the alignment of internal phase state.
Step 4 - Configure ODR and filters identically on both IMUs (i.e. same Gyro ODR, Accel ODR, AAF/UI filter settings and FIFO configuration). The reason for doing this is to minimize group delay as this will add to phase error, even with the same clock.
Step 5 - Enable sensors simultaneously for better phase alignment. Write PWR_MGMT0 on both devices back-to-back. Use SPI burst writes or GPIO-synchronization with chip select toggling. This will ensure valid sample is aligned to the same CLKN edge.
Step 6 - Use time stamps by enabling TMST_CONFIG to verify alignment by reading TMST_FSYNCH / TMST_FSYNCL. What you should see is identical or constant-offset time stamps. No long drift which confirms frequency lock. CLKIN ensures both accelerometer and gyroscope are aligned to the system clock.
Step 7 – Optional: FSYNC is not needed for frequency synchronization. You may want to use FSYNC if you want to tag frames (e.g. camera sync) or want a deterministic time stamp markers. FSYN does not replace CLKN, it aligns events, not sample clocks.
Summary: Here is how you synchronize two ICM-4268x IMUs.
The results will be a locked sampling frequency, deterministic, repeatable phase alignment with no long-term drift. The host and sensors share a common time base.