IIS2DLPC - Tap Detection Above +2g

[Jack9]

Hi,

I am working on an application where I need to generate an interrupt whenever a tap exceeds a certain acceleration threshold. I have already chose the IIS2DLPC for this purpose.

However, on page 58 of the datasheet, in Table 67 for the TAP_THS_Z register, I found the following:

"TAP_THSZ_[4:0] Threshold for tap recognition @ FS = ±2 g on Z direction."

I would like to clarify

• Does that mean that taps above 2g can't be detected. My application requires detecting relatively strong impacts, ideally up to 4 g, and preferably up to 8 g.

• If this is possible, could you advise on how to configure the device to achieve this?

• If it is not feasible, could you recommend any alternative accelerometers from your product line that would meet these requirements?

Thanks in advance!

 

 

[Federica Bossi]

Hi @Jack9 ,

As reported in the application note the TAP_THSX[4:0], TAP_THSY[4:0], TAP_THSZ[4:0] bits of the TAP_THS_X, TAP_THS_Y, TAP_THS_Z registers are used to select the unsigned threshold value used to detect the tap event. The value of 1 LSB of these 5 bits depends on the selected accelerometer full scale: 1 LSB = FS/32. The unsigned threshold is applied to both positive and negative high-pass filtered data.

1 LSB = FS/32, the thresholds are on 5 bits, so the maximum is 0x11111 = 31.

So, the maximum threshold that can be set is 31 * FS/32.

[Jack9]

Hi @Federica Bossi,

Thank you for your response. From your explanation, I understand that the maximum threshold can be configured as 31 × FS / 32. I would like to clarify if there is any restriction on the FS value when using the tap detection feature in the IIS2DLPC.

From the datasheet, my understanding is that tap detection requires FS to be set to ±2 g, although I may be mistaken. This requirement also seems to be indicated in another document, DT0101 Design Tip: Setting up single-tap and double-tap recognition with ST’s MEMS accelerometers.

 

Although this document describes the procedure for a different accelerometer, I assume the same requirement might also apply to the IIS2DLPC. 

In short, I would like to confirm whether it is possible to configure FS to a value other than ±2 g while still being able to use the tap detection feature.