MMA8652 Pulse Filtering Settings

[mross]

I am trying to set up pulse detection on an MMA8652 and have come across an ambiguity in the datasheet.

The description of register 0x0F HP_FILTER_CUTOFF says that bit 5 bypasses the High-pass Filter for Pulse processing when set, and bit 4 enables the Low-Pass filter when set. I can guess that when bit 5 is set and bit 4 is cleared, the raw sensor data will be used for the pulse processing. What isn't obvious though is what happens if bit 5 is cleared and bit 4 is set. Is that simply an invalid state, or does one of the HPF or the LPF take priority?

[reyes · Accepted solution]

Hi,

The pulse detection can be configured to run through the high-pass filter and also through a low-pass filter, which provides more customizing and tunable pulse detection schemes due to the HPF cutoff frequency customization.

Answering to your questions, correct, when bit 5 is set and bit 4 is cleared, the raw sensor data will be used for the pulse processing.

And when bit 5 is cleared and bit 4 is set, both high-pass and low-pass filters are enabled. For which case, you can set the high-pass filter cutoff frequency (bit0 and bit 1) for removal of the offset and slower changing acceleration data.

But, for simple pulse detection, my recommendation is to have the Low-Pass Filter enabled and high-pass filter bypassed, so, bit 4 Pulse_LPF_EN set (‘1’) and bit 5 Pulse_HPF_BYP set (‘1’).  

* When the high-pass filter is bypassed, the function behaves similar to the motion detection.

Have a great day,
Jose

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[mross]

Thank you, since there was no architectural diagram I was just assuming that there was a single LPF in the device that was being used for both the LPF and HPF.

I think I already know the answer to this one, but is there an appnote that discusses the pulse detection? I'm aware of the "Data Manipulation and Basic Settings" document mentioned in the datasheet, but it doesn't actually talk about the pulse detection at all.

[reyes]

Hi,

There is an application note that can be useful, just take in mind that it is designed for the MMA845x family, which is similar to the MMA865x, but it may differ in some registers, so, please take care of that.

You can find this document in the following link: https://www.nxp.com/docs/en/application-note/AN4072.pdf

Have a great day,
Jose

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