In a LIS2DTW12 accelerometer in sleep mode (SLEEP_ON bit = 1) able to detect short shocks (at least 10 ms long)?

[MLigg.1]

I'm developing a low power shock detector based on an SMT32L486 and a LIS2DTW12.

Normally the accelerometer is in sleep mode waiting to wake up. (SLEEP_ON bit = 1)

I want to detect short shocks (at least 10 ms long) but i understood in sleep mode the accelerometer ODR is only 12.5 Hz, it means a sample every 80 ms. In these conditions is the accelerometer able to wake up for 10 ms accelerations higher than the threshold?

Is it possible in sleep mode to increase the ODR to 200 Hz, in order to be still able to send a active or a single tap interrupt? And what about the power consumption?

Thanks for your answer.

Massimo Liggio

[Eleon BORLINI]

Hi Massimo,

I'm afraid that the SLEEP_ON mode is not configurable in terms of ODR, and is fixed to 12.5Hz and Low power mode. This because a change in ODR up to 200Hz would result in an increase of current consumption that would imply you won't be in a "sleep" condition any more.

You might use the TAP feature implementing a software routine the following way (see the datasheet, p.45): configure the device in LP mode, 200Hz, configure the tap threshold (and duration) to the desired value, wait until tap interrupt is raised and then change the operating mode into normal mode.

Let me please know if this solution is feasible for your purpose.

-Eleon

[MLigg.1]

Hi Eleon,

thanks for your hint, but in this way is it still possible to generare a wakeup interrupt? Or a sleep_change interrupt?

I'm using INT1 as tap or shock detection and INT2 as sleep_change detection in order to wake up the micro when the accelerometer starts to be moved and to stop it when the accelerometer recognizes it is inactive.

Massimo

[Eleon BORLINI]

Hi Massimo,

no, in this way you will generate the single TAP interrupt. For the SLEEP_ON feature you are limited to the 12.5Hz ODR...

Understand you are already using this feature for other purposes, so in this case I'm afraid it could not be possible to match the requirements of your app.

Did you already tested the device in SLEEP_ON mode with interrupt less than 12.5Hz ODR?

-Eleon

[MLigg.1]

Hi Eleon,

thank you for your response. The device works fine when I enable both SLEEP_ON and TAP interrupts. I think if the TAPs are relatively slow the accelerometer wakes up with the first movements, switches to an ODR of 200 Hz and detects the tap regularly. I will do some testing with defined shocks to evaluate the limits of this configuration.

Massimo