MMA8652 - Questions about Interrupts and ODR

[nkli]

Hello,

I had a question about how ODR affects interrupt speed on the MMA8652FC accelerometer. I noticed that there is a considerably large amount of delay when there is a lower ODR. My biggest concern is using the tap interrupt and getting an interrupt pulse from it while keeping a low power consumption.

At the moment, I am finding that 12.5Hz for ODR is extremely slow to respond. It feels like there is at least 3 seconds before there is a pulse generated from the MMA. Does this sound about right?

Also, what are ways to reduce power consumption of the MMA8652FC (not including reducing the ODR)? Is there a sleep mode other than the Auto-sleep option?

Thanks in advance.

[david_diaz]

Hello Nicole,

Thank you for writing.

The interrupt generation is totally related with the Output Data Rate.

For example, the image below shows the interrupt signal generated at Channel 2 every 80ms. Please note that I set the DATA_READY interrupt and ODR = 12.5 Hz (80ms).

So, the interrupt is related with the ODR. I recommend to modify the thresholds at the TAP detection.

In this case, indeed, you can reduce the power consumption setting the ODR. Besides that, you can set the power mode at the CTRL_REG2 register.

Please review the table below in order to know the different power consumption:

I hope this information helps.

Regards,

David

[anthonyduhamel]

Hello Nicole,

Have you read the application AN3919 note on the tap detection? It can be helpful for you application.

Per the document, the time period of a tap event if about 10ms.  

With an ODR of 12.5Hz, your time resolution is equal to 8.3ms. This subsampling does not allow to efficiently catch the tap event (it really depends on the phase beteween the tap event and the MMA8652 clock).

I do not recommand to use this feature at 12.5Hz. Usually we use 200 Hz for tap detection. 

You may use the transient detection mechanism with the high pass filter on, but at this ODR, I'm not expecting good results.

I supose you configured the MODS (CTRL_REG2) to 0b11 (low power). There is no other way to reduce the current consumption without changing the ODR. Eventually, you can trig the Auto-Wake up with another interrupt fonction. I mean, you can set the ASLP rate to a lower frequency, when it detectes a motion, it switches to wake mode but you will have a time latency between the wake up event and the tap detection enablement. 

Can you give more information/details about your usecase/application. Where do you want to integrated the MMA8552?

Regards,

Anthony