Issue with IMU Sensor (LSM6DSV16X) Sample Rate in Sensor Hub Mode

[SaeedZadeh]

I am using an IMU sensor in sensor hub mode (LSM6DSV16X, LIS2MDL) with the FIFO interrupt interval set at 30 Hz. The data is sent over BLE. It works perfectly when using RTT, but when I disable debug mode, the sample rate drops to around 1 Hz.

Could someone please help me resolve this issue?

Thank you.

[SaeedZadeh]

Thank you for your reply. When the IMU's FIFO is full, an interrupt will occur on the interrupt pin. The interrupt rate will decrease to 1 Hz as if something changes or is missed when I disable the RTT configuration.

[Edvin]

So what triggers the IMU readings in the first place? Do you have a timer set up at 30Hz or something?

What HW are you running this on? nRF5340DK? Or do you have a custom board?

[SaeedZadeh]

Sure, I recorded the signal while the RTT log was enabled, and after some moments, I disabled it.

This is the whole signal; please notice the red line shows the FIFO interrupt events:

 

This picture shows the beginning and end of one burst of sensor output while RTT is enabled; It doesn't show the middle signals as they are just the IMU data values:

And this picture belongs to the signals when the RTT log was disabled:

As you can see, everything is the same, but the output signal rate.

the 1B register is FIFO_STATUS1 register

[Edvin]

Hello,

So this is a I2C device. I assume the nRF is the I2C Master, right? Unless the sensor is the I2C master, then all the read events must be triggered by the nRF, and this means that you must use some event to trigger the next transmission. 

Is it possible to share some snippets/files showing how you set up and communicate with the sensor?

[SaeedZadeh]

I appreciate your help, please see the link below:

https://github.com/Saeed-ak/my_rip.git

[Edvin]

Hello,

So I ran your application. Thanks for including the IMU_SIMULATION definition. It makes these things a lot simpler.

Just to clarify, you see the same behavior when setting IMU_SIMULATION to 1 as well?

When you say "disable logging", you mean setting these two in your prj.conf?

CONFIG_SERIAL=n
CONFIG_LOG=n

If so, I see nowhere near 1 second. 

I do, however, see this when I enable notifications on my central:

So it is slower, but it is also actually sending notifications, so I expect there to be some delays. 

nrf5340_imu_adc_led_ble.zip

Can you please test the attached project, and measure pin P0.05 using the logic analyzer? What does it look like? (if you don't use the actual sensor, but the simulated sensor). Or does the issue only occur when you set IMU_SIMULATION to 0?

Best regards,

Edvin

[SaeedZadeh]

Thank you for your reply.

Edvin said:

you see the same behavior when setting IMU_SIMULATION to 1 as well?

yes

Edvin said:

When you say "disable logging", you mean setting these two in your prj.conf?

I meant, when I connect to RTT. When it connects, I receive a proper signal; once I disconnect it, the frequency of interrupts significantly drops. Here:

Now, during testing, I just realized that the same behaviour is present even in simulation mode.

Edvin said:

Can you please test the attached project, and measure pin P0.05 using the logic analyzer?

There is no signal on this pin in any configuration.

Edvin said:

r does the issue only occur when you set IMU_SIMULATION to 0?

As I mentioned above, yes, it occurred in both modes. I can see the data rate changes on another device connected to this device through BLE.

Best,

[Edvin]

Hello,

SaeedZadeh said:

There is no signal on this pin in any configuration.

Did you connect to the device with a BLE central? It will not start toggling until ble_send() is called.

I struggle to understand your snippets from the logic analyzer. I am sorry. What is the D2 line in your plot? If it isn't already, can you please try to toggle a pin every time you call ble_send()?

BR,
Edvin

[SaeedZadeh]

Yes, I connected to the BLE central. 

D2 is the interrupt pin of the IMU, which is connected to the nrf53dk.

[Edvin]

What direction does that pin go? Is it an input or output for the nRF53?

Edvin said:

If it isn't already, can you please try to toggle a pin every time you call ble_send()?

[SaeedZadeh]

It is input.

I also realized that even when I activated the IMU_SIMULATION mode, my problem persisted. This means that the nRF kept sending the ADC value through BLE to the BLE central at the same rate, whether the IMU was activated and RTT was connected or not. (my judgment reference is the serial output of the central BLE that I monitor on the PC.)

[Edvin]

Ok, why the sensor behaves differently I don't know. You have to refer to it's data sheet or contact the manufacturers. 

SaeedZadeh said:

I also realized that even when I activated the IMU_SIMULATION mode, my problem persisted. This means that the nRF kept sending the ADC value through BLE to the BLE central at the same rate, whether the IMU was activated and RTT was connected or not.

I don't understand. You say that the issue persisted, and that the ADV values are sent at the same rate? Isn't the issue that the ADV values are sent at a slower rate when you disconnect RTT?

Is it an option to use a k_timer to trigger the readings at a given frequency, and a work handler to actually read the samples from your sensor:

https://docs.nordicsemi.com/bundle/ncs-latest/page/zephyr/kernel/services/timing/timers.html#using_a_timer_expiry_function

Try this and see if this is able to keep the timing consistent.

Best regards,

Edvin