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3.8.9
Arming Function
MMA68xx provides the option for an arming function with 3 modes of operation. The operation of the arming function is se-
lected by the state of the A_CFG bits in the DEVCFG register.
Reference Section 4.5 for the operation of the Arming function with exception conditions. Error conditions do not impact prior
arming function responses. If an error occurs after an arming activation, the corresponding pulse stretch for the existing arming
condition will continue. However, new acceleration reads will not update the arming function regardless of the acceleration value.
3.8.9.1
Arming Function: Moving Average Mode
In moving average mode, the arming function runs a moving average on the offset cancelled output of each acceleration axis.
The number of samples used for the moving average (k) is programmable via the AWS_Xx[1:0] and ARM_Yx[1:0] bits in the
ARMCFGX and ARMCFGY registers. Reference Section 3.1.8 for register details.
ARM_MA n = (OC n + OC n-1 + ... + OC n+1-k )/k
Where n is the current sample.
The sample rate for each axis is determined by the SPI acceleration data sample rate. At the falling edge of CS for an accel-
eration data SPI response, the moving average for the associated axis is updated with a new sample. Reference Figure 25 . The
SPI acceleration data sample rate must meet the minimum time between requests (t ACC_REQ_x ) specified in Section 2.5 .
The moving average output is compared against positive and negative 8-bit thresholds that are individually programmed for
each axis via the ARMT_Xx and ARMT_Yx registers. Reference Section 3.1.9 for register details. If the moving average equals
or exceeds either threshold, an arming condition is indicated, the ARM_X or ARM_Y output is asserted for the associated axis,
and the pulse stretch counter is set as described in Section 3.8.9.4.
The ARM_X or ARM_Y output is de-asserted only when the pulse stretch counter expires. Figure 25 shows the arming output
operation for different SPI conditions.
ARMT_xP[7:0]
AWS_xP[1:0]
Positive
Moving Average
Offset Cancellation
OffCanc_ARM_x[10:0]
Pulse Stretch
Gating
I/O
ARM_x
Negative
AWS_xN[1:0]
Moving Average
ARMT_xN[7:0]
APS_x[1:0]
Figure 23. Arming Function Block Diagram - Moving Average Mode
MMA68xx
Sensors
Freescale Semiconductor, Inc.
37
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