± 2g / 4g / 8g / 16g Tri-axis Digital
Accelerometer Technical
Reference Manual
PART NUMBER:
KX132-1211
Rev. 1.0
31-Jul-2019
36 Thornwood Dr.
–
Ithaca, NY 14850
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–
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54
of
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reset of the reference sample. At this point, the algorithm starts evaluating the difference between the current and the
previous acceleration samples again. On the 9
th
clock cycle, the difference has exceeded the threshold. The sample
evaluated on the 8
th
clock cycle becomes a new
reference
sample for the algorithm. The debounce counter starts the
count on the next clock cycle (9
th
). During the next 8 clock cycles (9
th
-16
th
), the acceleration difference between each
new sample and the
references
sample (8
th
clock cycle) has remained above the threshold and the debounce counter
has incremented at each clock. Once the count value reached the WUFC value of 8, the wake-up interrupt was issued
on the next clock cycle (17
th
clock cycle). In the example, since the interrupt was set to active low, the interrupt line was
pulled low to indicate an interrupt. The operation of the back-to-sleep functionality is equivalent to the wake-up function.
Figure 4:
Latched Motion Interrupt Response in the Relative Threshold Mode
In cases when acceleration changes are gradual relatively to the wake-up, the motion may not get detected because
the difference between consecutive samples is below the threshold value. In such cases, consider reducing the motion
wake-up sampling rate as set by OWUF<2:0> bits in CNTL3 register or using the absolute threshold mode. Consider
the motion shown in the Figure 5. Even though the difference between the initial acceleration measured on the 2
nd
clock
(
init
) and final sample measured at the 7
th
clock (
initi+g2)
is higher than the threshold, the difference between
consecutive intermediate samples (4
th
-3
rd
, 5
th
-4
th
, 6
th
-5
th
, 7
th
-6
th
) was below the threshold. Thus, the condition was never
met to start the debounce counter.
Figure 5:
Motion Interrupt Response in the Relative Threshold Mode with Slow Acceleration Transition
